Software Configuration Management (SCM) Security

Software Configuration Management (SCM) Security
By David A. Wheeler
Security and Aegis
By Peter Miller
Introduction Introduction
Software development is often supported by specialized programs called "Software Configuration Management" (SCM) tools. SCM tools often control who can read and modify the source code of a program, keep history information (so that people can find out what changed between versions, and who changed them), and generally help developers work together to improve a program under development. The left column is David Wheeler's essay about software configuration management and security. This column contains a running commentary on how Aegis achieves many of the desirable security goals David outlines.
Problem is, the people who develop SCM tools often don't think about what kind of security requirements they need to support. This mini-paper describes briefly the kinds of security requirements an SCM tool should support. Not every project may need everything, but it's easy to not notice some important requirements if you don't think about them. There are two basic types of SCM tools, "centralized" and "distributed"; the basic security needs are the same, but how these needs can be handled are different between the two different types. I'm primarily concentrating on basic SCM tools (like CVS, Subversion, GNU Arch, Bitkeeper, Perforce, and so on). Clearly related tools include build tools, automated (regression) test tools, bug tracking tools, static analysis tools, process automation tools, software development tools (such as editors, compilers, and IDEs), and so on. Aegis was designed from the very beginning with security in mind. Indeed, for some time it provided rather more security than open source developers needed or wanted.

Recent developments have provided additional configuration options to allow a more relaxed security profile than is provided by default.

The Security Basics The Security Basics
Fundamentally, there are some basic (potential) security requirements that any system needs to consider. These are: Here are the security features that Aegis is able to provide:
  • confidentiality: are only those who should be able to read information able to do so?
  • Confidentiality: projects may be configured so that only authorized staff are able to read their source files or access their history. It is even possible to conceal a project's existence from other users on the same computer.
  • integrity: are only those who should be able to write/change information able to do so? This includes not only limiting access rights for writing, but also protecting against repository corruption.
  • Integrity: Aegis has separate ACLs for developers, reviewers and integrators. Various operations during the lifetime of a change set are limited to one or other of these roles.
  • Integrity: It is possible (and recommended) that the project repositories (history and baselines) be read-only to all project staff. The rest of this discussion will assume this facility is being used. All operations which modify the repository are mediated by Aegis.
  • Integrity: The meta- data files cannot be edited by project staff.
  • Integrity: The history files cannot be edited by project staff.
  • availability: is the system available to those who need it? (I.E., is it resistant to denial-of-service attacks?)
  • Availability: Aegis relies on the host computer's facilities for this, including backup of its meta-data and repository.
  • identification/authentication: does the system safely authenticate its users? If it uses tokens (like passwords), are they protected when stored and while being sent over a network, or are they exposed as clear text?
  • Authentication: Aegis relies on the host operating system for this. Once the operating system has authenticated a user, Aegis trusts the operating system.
  • Authorization: Aegis has separate ACLs for each project for each role (developer, reviewer, integrator). Only authorized users may advance a change set through the process.
  • audit: Are actions recorded?
  • Audit: Aegis records all actions by all users who advance a change set through the process, including the user's identity, what they did and a time stamp.
  • non-repudiation: Can the system "prove" that a certain user/key did an action later?
  • Non-repudiation: Because Aegis relies on the host operating system to authenticate users, the non-reputation features of the operating system carry over to Aegis.
  • self- protection: Does the system protect itself, and can its own data (like time stamps) be trusted?
  • Self-protection: The recommended mode of operation, where the projects repository (history and baselines) are read-only to the entire project team, means that all changes to the repository are mediated by Aegis. Thus, Aegis can trust that its meta-data is untainted (it can't defend against root privilege).
  • trusted paths: Can the system make sure that its communication with users is protected?
  • Trusted paths: Aegis has a number of features which facilitate geographically distributed development. By using tools such as OpenPGP or GnuPG when change sets are in transit, as much or as little protection as desired can be achieved.
An SCM has several assets to protect. It needs to protect "current" versions of software, but it must do much more. It needs to make sure that it can recall any previous version of software, correctly, as well as the audit trail of exactly who made which change and when. In particular, an SCM has to keep the history immutable - once a change is made, it needs to stay recorded. You can undo the change, but the undoing needs to be recorded separately. Very old history may need to be removed and archived, but that's different than simply allowing history to be deleted. Aegis has complete control of the history of a project and the associated meta-data. All change sets are preserved forever.

Most aspects of Aegis' meta-data are immutable. Only project administrators may change the description of a change once it has been committed to the repository (developers are notoriously bad at accurately describing their changes); project administrators cannot alter any information about the file histories or the various users and time stamps the change set has accrued.

The Threats The Threats
Okay, so what are the potential threats? These vary, and not all projects will worry about all threats. Nevertheless, it's easier to provide a list of threats and the counter-measures an SCM should support. Aegis was designed from the ground up to cope with a number of threats.
Individual projects may choose to not employ a given counter-measure, since they may decide that's not a threat for them. For example, open source software (OSS) projects may decide that there's no "threat" of unauthorized reading of software, since the code is open to reading by all. However, that may not always be true - many OSS projects hide changes that reveal security vulnerabilities until the new version is ready for deployment. Thus, it's difficult to make simple statements like "projects of type X never need to worry about threat Y". Instead, it's simpler to list some potential threats, and then projects can decide which ones apply to them (and configure their SCM system to counter them). Aegis provides the ability to customize many aspects of the process which change sets travel through to be included in the project repository.

The default configuration is medium paranoid. It is possible to increase that to extremely paranoid (or extremely officious, depending on your perspective), or weaken it to merely be helpful.

Outsiders without privileges Outsiders without privileges
An outsider (not a developer or administrator) may try to read or modify assets (software source code or history information) when they're not authorized to do so. SCM systems should support authorization (like login systems), and support a definition of what unauthorized users can do. An SCM system should support configurations that allow anonymous reading of a project and/or its history, since there are many cases where that's useful. However, SCMs should also support forbidding anonymous read access. That's even true for OSS projects, since as I noted above, sometimes OSS projects want to hide security fixes until they're ready for deployment. By using the UNIX groups facility (see group(5) for more information) it is possible to limit the read access to a project to members of a single group.

However, this leaks the existence of the project. By using the AEGIS_PATH environment variable, telling Aegis where to look for projects, it is also possible to conceal the existence of a project.

Aegis has separate ACLs to developers, reviewers and integrators, limiting the number of users who can modify the project - but even then, they must follow the process.

Normally unauthorized users shouldn't be allowed to modify a source repository, so an SCM should support that (and should make that the default). In rare cases, it's possible to imagine that even this constraint isn't true, especially if the SCM tool is designed to be used for resources other than source code. Most Wiki systems such as Wikipedia allow anonymous changes; they work instead by protecting the history of changes so that everyone will know exactly what's changed, instead of preventing writing of the primary data. Such approaches are rare for software code; for example, the Wikipedia software itself (as stored in its trusted repository) can only be changed by a few privileged developers. However, it is conceivable that software documentation and code would be maintained by the same SCM software, and perhaps a few projects would allow anyone to update the documentation as long as all changes were tracked and could be easily reversed. Aegis is very strict about which users are authorized to create and modify change sets. If you are not in the appropriate ACL, you may not perform the action.

It would be possible (although this isn't the case at present) to add code to Aegis to allow "anybody" to be a change set developer. It is probably most undesirable to allow "anybody" to be a code reviewer or integrator.

Aegis breaks the traditional "commit" step into several pieces. Large project are able to be configured to require a user other than the developer to perform the code review step (this is the default). Thus, it is safe for naive developers to modify any source file they like, but they have to get it past a code reviewer before it will appear in the repository. There are facilities in Aegis to require specific code reviewers for specific portions of the code.

The underlying identification and authentication system (the login system) can use intrusion detection systems to detect likely attempts to forge privileges (e.g., by detecting password guessing attacks, or detecting improbable locations of a login). The underlying login system could also support enabling limits (e.g., delays after X login attempts, or only permitting logins from certain Internet Protocol address ranges for certain developers). However, these mechanisms need to not create a denial-of-service attack; otherwise, an attacker might try to forge logins not to actually log in, but to prevent legitimate users from doing so. Aegis relies on the host operating system for authentication services. The extent to which the host operating system can detect attacks is the extent to which project managed by Aegis are safe or at risk.
Non-malicious developers with privileges Non-malicious developers with privileges
An SCM system should support protected logins (e.g., if it uses passwords, it should protect passwords during transit and while they're stored). Once users are authenticated, an SCM system should be able to limit what users can do based on the authorization that's implied. Aegis relies on the host operating system for authentication services. Once authenticated, individual users may be authorized to perform different actions, based on simple ACLs. Each project has separate ACLs for the various roles.
SCM systems could usefully limit reading to particular projects, say. Limiting reading of specific files inside a project can be useful, but it often isn't as useful inside a branch developers must access because developers often need the entire set of files to develop (e.g., to recompile something). But limiting who can read changes in certain branches could be vital for some projects. For example, it is common for security vulnerabilities to be reported to a smaller group of people than the entire development staff, and for the patch to be developed by specially trusted developers without full knowledge of all developers. This is particularly true for open source software projects, but it's also sometimes true for other projects. This kind of functionality can also be important for projects such as military projects with varying degrees of confidentiality; most of the program may be "unclassified", but with a poor or stubbed algorithm; there may be a better classified algorithm, but it will need to be maintained separately. Ideally, the SCM should be trustworthy enough to protect that data, though in practice such trust is rarely granted; an SCM should instead gracefully handle importing the "unclassified" version and automatically merging the "classified" data on equipment trusted to do so. Aegis does not provide this facility on a per-file or per-directory bases. It is possible to vary staff roles per branch, but this does not include read access.
Limiting writing of specific files inside a project can be much more useful, since in some projects some users "own" certain files. In many situations it doesn't make sense either, but an SCM system should still support limiting which developers can make which changes. Aegis breaks the traditional "commit" step into several pieces. Large project are able to be configured to require a user other than the developer to perform the code review step (this is the default). Thus, it is safe for naive developers to modify any source file they like, but they have to get it past a code reviewer before it will appear in the repository. There are facilities in Aegis to require specific code reviewers for specific portions of the code.
Malicious developers with privileges (and attackers with their credentials) Malicious developers with privileges (and attackers with their credentials)
An area often forgotten by SCM systems is handling malicious developers. You know, the ones who intentionally insert Trojan horses into programs. Denying they exist doesn't help; they do exist. And even if they didn't, there's no easy way for an SCM to tell the difference between an authorized malicious developer and an attacker who's acquired an authorized developer's credentials. Never ascribe to malice what you can ascribe to stupidity. Developers (particularly, tired and overworked developers) frequently do stupid things. If you can think of a malicious way to subvert a system, some user will do it by accident and not know he did it.
A malicious developer might even try to make it appear that some other developer has done a malicious deed (or at least make it untraceable). They can use their existing privileges to try to gain more privileges. A malicious developer might try to modify the data used by a CM system so that it looks like someone else made the change (e.g., provide someone else's name in a ChangeLog entry). A malicious developer might try to modify a CM "hook" to make it appear that some other developer has inserted malicious code (perhaps to avoid blame or frame the other developer). A malicious developer might modify the build process, e.g., so that when another developer builds the software, the build system attempts to steal credentials or harm the developer. Aegis trusts the operating system to authenticate users. Once authenticated, users do not have write access to the meta-data or history files of any change set. The vast majority of "hooks" are in files controlled by Aegis' process; to maliciously change one requires a conspiracy of developer, reviewer and integrator.
Since developers have the privileges to read and change data, malicious developers (and attackers with their credentials) are harder to counter. But there are counter-measures that can be used against them. Here are some reasonable measures: While developers have the privileges to read source files, and the ability to change them in their private work areas, they are unable (by themselves) to modify file histories or change set meta-data. Here are some of Aegis' counter-measures:
  • Make sure that developers can't corrupt the repository. As a counter-example, GNU Arch allows developers to share a writable directory as a repository. That's very convenient, but if you're worried about malicious developers, that's not enough; a malicious developer could easily remove data or corrupt it in such a way that it'd be hard to tell who caused the problem (there's current effort to create an "archd" server that would probably counter this problem).
  • The recommended Aegis configuration has the history files and the project meta-data read-only to the entire development team, even users authorized to integrate change sets into the repository. All operations which advance a change set along the process are logged, including the final one which actually alters the change set's files' histories.
  • Make sure that all developer actions are logged in a non-repudiatable, immutable way. That way, even if someone makes a change, it's easy to see who made what changes, in any time in the future. That "someone" may be a malicious developer, or an attacker with the credentials (e.g., cryptographic keys) of a developer -- but in either case, once you find out who did a malicious act, the SCM should make it easy to identify all of their actions. In short, if you make it easy to catch someone, you increase the attackers' risk... and that means the attacker is less likely to do it. In practice, this can be done by requiring that all changes be cryptographically signed by each developer. Implied here is that there is an easy way to undo those changes; after all, if it's easy to identify exactly what a developer did, they can be undone.
  • All operations which advance a change set along the process are logged. There are no facilities for removing this meta- data, and the meta-data files themselves are read-only to the entire project team.
  • The non-repudiation of logged change set events is limited to the ability of the operating system to provide non- repudiatable user authentication.
  • Make sure all developer actions can be easily reviewed later. A simple action to show exactly what's been changed recently will make it easy for new changes to be reviewed - and possibly set off alarms.
  • Aegis provides numerous ways to review change sets and/or files. You can obtain a listing of all historical events for a change set. You can obtain a listing of all historical events for a file. You can obtain a listing of all change sets applied to a project to data. You can get "blame listings" for files.
  • Have tools to record and/or require others' review. If you really want to make sure that malicious code doesn't get through, the best method known is to make sure that some other person (who is unlikely to be colluding) reviews the code. Thus, ways to cryptographically sign that a person reviewed anothers' changes can be helpful, as long as the reviewer's signature can't be forged, and as long as the signature clearly indicates what was reviewed. A review could be at a brief "I briefly scanned for malicious code" all the way to "I deeply analyzed every line for correctness", so the SCM tool should support recording to what level the review occurred too.
  • By default, Aegis requires code reviews; it is possible to configure them away. It is possible to require several code reviewers for a change set, or specific code reviewers for portions of the code. The review policy is highly configurable.
  • There are several "hooks" that can be used to perform automatic checking for change sets: the build hook, the diff hook and the review policy hook.
  • Support automated checking before acceptance, including detection of suspicious/malicious changes. An SCM system should make it possible to enforce certain rules before accepting a change (at some level): such as enforcing formatting rules, requiring a clean compile, and/or requiring a clean run of a regression test suite (in a suitably protected environment). It should be possible to watch changes to find "suspicious" changes: the first time that developer has modified a given file, code that looks a like a Trojan horse, formatting/naming style that's significantly different than this developer's normal material, attempts to send email or other network traffic during a code build, and so on. This is basically intrusion detection at the code change level. It should also be possible for an automated process to quickly check for hints of "stolen" code before accepting anything (e.g., to detect copyright-encumbered code), by calling to programs such as Eric S. Raymond's comparator.
  • Aegis provides the ability to prevent a change set from advancing to code review until it builds successfully.
  • Aegis provides the ability to prevent a change set form advancing without a unit test and the test must pass.
  • Aegis provides the ability to require a bug-fix change set's unit test to fail against the unaltered code baseline, to confirm that the test accurately reproduces the bug.
  • Aegis makes it simple to run all of these accumulated tests against a change set's private work area to confirm that the change set will not break anything.
  • It is possible to have Aegis suggest suitable tests to run for the files in a change set, based on the correlation between source files and accompanying test files in previous change sets.
  • Support authentication/cryptographic signature key changes and re-signing. No matter what protection is put in place, a developer's secrets (e.g., their login passwords or private keys) may be acquired by an attacker. Thus, an SCM (along with its support environment) need to support changing such secrets. In particular, it may be useful to "cycle" developer private keys, having developers switch to new private keys, ensuring that the old keys will not be accepted for newer changes, and possibly destroying all copies of the older private keys (so that they cannot be stolen by anyone). Since private keys may be compromised, once such a compromise has been detected, it should be possible to invalidate the compromised keys and re-sign data (once it's checked) with new cryptographic keys. This is yet another reason to support multiple signature keys (in addition to supporting multi-person review).
  • Aegis depends on the host operating system for authentication services.
  • When transmitting change set between repositories, it is possible to use OpenPGP or GnuPG to provide adequate security.
  • On login, acquisition, and commit, report the "last time" and source location (e.g., IP address) where reading and writing (committing) were performed. Although this doesn't deal with a malicious developer, it does increase the likelihood that an attack using stolen credentials will be detected. After all, the developer is mostly likely to know the last time that they read from and wrote to some repository, so they'll be able to detect when someone else forges their identity. Ideally, this would be resistant to repository attacks.
  • Aegis depends on the host operating system for authentication services.
On April 11, 2004, Dr. Carsten Bormann from the University of Bremen sent me an email about a specialized attack that he terms the "encumbrance pollution attack". In an encumbrance pollution attack, the attacker inserts material that cannot be legally included. To understand it, first imagine an SCM with perfectly indestructible history. The attacker steals developer credentials, or is himself a malicious developer, and checks in a change that contains some encumbered material. "Encumbered" material is simply material which cannot be legally included. Examples include child pornography, slanderous/libelous statements, or code which has copyright or patent encumbrances. This could be very advantageous, for example, a company might hire a malicious developer to insert that company's code into a competing product, and then sue the competitor for copyright infringement, knowing that their SCM system "can't" undo the problem. Or a lazy programmer might copy code that they have no right to copy (this is rare in open source software projects, because every line of code and who provided it is a matter of public record, but it proprietary projects do have this risk). Any SCM can record a change that essentially undoes a previous change, but if the history is indestructible and viewable by all, then you can't get rid of the history. This makes your SCM archive irrevocably encumbered. This can especially be a problem if the SCM is indestructibly recording proposals by outsiders! An SCM system could be designed so that a special privilege allowed someone to completely deletion the history data of illegal changes, of course. However, if there are special privileges to delete history data, it might be possible to misuse those privileges to cause other problems.

One mechanism for dealing with an encumbrance pollution attack is to allow specially-privileged accounts to "mask" history elements; i.e., preventing access to certain material by normal developers so that it's no longer available, so that the material isn't included in later versions (essentially it works like an "undo" against that change). However, a "mask" would still record the event in some way so that it would be possible to prove that the event occurred at a later time. Perhaps the system could record a hash of the encumbered change, allowing the encumbered material to be removed from the normal repository yet proving that, at one time, the material was included. A "masking" should include a cryptographic signature of whoever did the masking. This mechanism in particular requires careful design, because the mechanism should be design so that it doesn't permit other attacks.

  • By using Aegis' code review feature, it is possible to mitigate the risk of an "encumbrance pollution attack". If the change set is unacceptable, it will be (should be) caught by the code reviewer.
  • Until a change set is integrated, its history and meta- data are not immutably encumbered.
  • The meta-data files (and often the source file) are all simple text files. It would be possible (via root) to edit the offending portions away.
Most SCM systems have multiple components, say, a client and server. Even GNU arch, which can use a simple secure FTP server as a shared repository, has a possible server (the FTP server). Clients and servers should resist attack from other potentially subverted components, including loss of SCM data. The Aegis support for geographically distributed development includes the whole process for each change set. This means that synchronizing with a remote repository is not a back door to your repository.
Repository attacks Repository attacks
Many repositories have themselves undergone attack, including the Linux CVS mirror, Savannah, Debian, and Microsoft (attackers have acquired, at least twice, significant portions of Windows' code). Thus, a good SCM should be able to resist attack, even when the repository it's running on subverted (through malicious administrators of a repository, attacker root control over a repository, and so on). This isn't just limited to centralized SCM systems; distributed SCM systems still have the problem that an attacker may take over the system used to distribute someone's changes. Aegis depends on the host operating system for limiting access to the repository history and meta-data. Aegis has no defense against a compromised root account making direct access to the history and meta-data, without executing Aegis.
An SCM should be able to prevent read access, even if the repository is attacked. The obvious way to do this is by using encrypted archives. But there are many variations on this theme, primarily in where the key(s) are stored for decryption. If the real problem is just to make sure that backup media or transfer disks aren't easily read, the key could simply be stored on a separate (more protected) media. The archive keys might only be stored in RAM, and required on boot up; this is more annoying for boot up, and an attacker is likely to be able to acquire the data anyway. The repository might not normally have the keys necessary to decrypt the archive contents at all; it could require the developer to provide those keys, which it uses and then destroys. This is harder to attack, but a determined adversary could subvert the repository program (or memory) and get the key. Another alternative is to arrange for the repository to not have the keys necessary to decrypt the archive contents at any time. In this case, developers must somehow be provided with the keys necessary to do the decryption, and essentially the repository doesn't really "know" the contents of the files it's managing! Aegis has no support for encrypted meta-data, and limited support for encrypting history files. Add to this the problem that a change set must be approved by a code reviewer, who may not have the developer's keys.

Aegis was designed long before this concept evolved. Its meta-data is not in a form which allows it to be placed in an immutable file, with each update to the meta-data being placed in another immutable file.

Portions of the meta-data are immutable, but because it is stored in the same file as some changeable data (for example, where the change set is in the process) digital signatures are problematic.

It is possible to encrypt change sets when they are in transit between repositories, using OpenPGP or GnuPG.

Preventing write access when an attacker controls a repository is a difficult challenge, especially since you still want to permit legitimate changes by normal developers. Since the attacker can modify arbitrary files in this case, the goal is to be able to quickly detect any such changes: Aegis was not designed to do this.
  • Cryptographic signing of changes can help significantly here, since this makes it possible to detect changes by anyone other than the authorized developers. Clearly, the list of public keys needs to be protected; this can be protected in part by ensuring that the list is visible to all developers, and having tools automatically check that the public listed key is correct (each developer's tool checks that the key listed is really that developer's key).
  • Aegis was not designed to do this.
  • Change set chaining can help detect problems (including unintentional ones). Basically, as changes are made, a chain recording those changes can be recorded and later checked. This is typically done using cryptographic hashes, possibly signed so you know who verified the chain. Note that this is also useful for detecting accidental corruption.
  • Aegis was not designed to do this.
  • Automated tools to detect if "my" change has been altered. Any given developer will know what changes they checked in. So, record that information locally/separately, and check it later. That way, someone can modify the repository to remove the latest security fix, but the developer of the change can quickly tell that it's been removed.
  • Aegis was not designed to do this.
  • Immutable backups, and tools to check them, can help as well. If a repository's history is changed, that change can be compared with backups. Be careful that a corrupted tool won't create misleading backups, and make sure that the repository can't give one view to backup tools, and another view to whoever actually takes and uses the program.
  • Aegis relies on the host operating system for backups. If the host operating system is capable of cryptographically signed backups, use them.
  • Simple, transparent formats can help make it harder to hide attacks. Data that is stored in simple, well-understood formats that can be analyzed independently (e.g., a signed tar file of patches) tend to be more resistant to attack than data structures that presume that no other process will manipulate the data contents (e.g., typical databases).
  • Aegis' meta-data is in simple text files. These can be parsed by external verification programs if desired. (Aegis pre-dates XML by over a decade; these days XML would have been a good choice.)
Related Work Related Work
[This section omitted. See the original paper.] Omitted.
Conclusions Conclusions
All of this can't prevent all attacks. But such an SCM system can make the attacks much harder to perform, more likely to be detected, and make detection much more rapid. Here are some examples: Aegis was designed to prevent several classes of attacks. The process used by Aegis is designed to make defective change sets (including malicious change sets) more likely to be detected. Here are some examples:
  • A malicious developer could insert a few lines into a build process that said "when you compile, email to me your private key data" - then, once they had the private key, remove that line, and then forge other changes as that unsuspecting developer. But an SCM system with all of the capabilities above would make it much harder to hide this. The change with these malicious instructions would be clearly labelled as from that developer, and later changes would be labelled as being from that developer or one of the compromised systems - and removing the change later would record yet another change that might be detected.
  • In a properly configured project, in order for malicious code to be inserted into the build system, the code reviewer has to be compromised, negligent or the attacker must compromise two users' accounts, not just one.
  • The second change set must also pass through the process, increasing the risk that something will be noticed.
  • A malicious attacker might take over the repository, and repeatedly remove a critical patch to a security vulnerability. Still, the removal could be detected by the creator of the patch, and actions such as changing to a different repository could be performed. Trying to change older copies would likely be detected by chaining and comparisons with backups.
  • As mentioned above, Aegis has no defense against a compromised root account, however this is not a back-door into any other machine via Aegis' support for geographically distributed development. If root isn't compromised, rescinding a previous critical security change set requires at least the help of a reviewer.
It's my hope that SCM systems will have more of these capabilities in the future. Aegis has been capable of of preventing many of the described attacks for over a decade. It is a mature SCM with users including medical equipment vendors with strict FDA compliance requirements.

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