GHSA-g8r4-p96j-xfxc CVE-2021-29440
Grav's Twig processing allowing dangerous PHP functions by default
High
Packagist
- Severity
- 8.4 / 10
- High · CVSS 3.1
- Exploit chance (EPSS)
- 29.90%
- Chance of exploitation in the next 30 days. Higher than 98.20% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Weakness
- CWE-94 Improper Control of Generation of Code ('Code Injection')
- Fix
1.7.11- Upgrade getgrav/grav to this version or later
Description
Impact
Twig processing of static pages can be enabled in the front matter by any administrative user allowed to create or edit pages. As the Twig processor runs unsandboxed, this behavior can be used to gain arbitrary code execution and elevate privileges on the instance.
Patches
The issue was addressed by preventing dangerous functions from being called in Twig templates. A configuration option has been added to manually allow arbitrary PHP functions (system.twig.safe_functions) and filters (system.twig.safe_filters).
Futures major versions of Grav may disable this functionality by default.
Workarounds
Blocking access to the /admin path from untrusted sources will reduce the probability of exploitation.
References
- https://portswigger.net/research/server-side-template-injection
- https://blog.sonarsource.com/grav-cms-code-execution-vulnerabilities
For more information
If you have any questions or comments about this advisory, you can contact:
- The original reporters, by sending an email to vulnerability.research [at] sonarsource.com;
- The maintainers, by opening an issue on this repository.
Affected versions
Packagist
| Package | Vulnerable | Fixed in |
|---|---|---|
| getgrav/grav | < 1.7.11 |
1.7.11 |
How to fix
Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.
getgrav/grav Packagist → 1.7.11
- composer
composer require getgrav/grav:^1.7.11
Severity breakdown
CVSS 3.1 8.4
CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:H/I:H/A:H
- Attack vector
- Network
- Attack complexity
- Low
- Privileges required
- High
- User interaction
- Required
- Scope
- Changed
- Confidentiality
- High
- Integrity
- High
- Availability
- High
Weakness
CWE-94: Improper Control of Generation of Code ('Code Injection')
The product constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment.
How to prevent it
- Architecture and Design: Refactor your program so that you do not have to dynamically generate code.
- Architecture and Design: Run your code in a "jail" or similar sandbox environment that enforces strict boundaries between the process and the operating system. This may effectively restrict which code can be executed by your product. Examples include the Unix chroot jail and AppArmor. In general, managed code may provide some protection. This may not be a feasible solution, and it only limits the impact to the operating system; the rest of your application may still be subject to compromise. Be careful to avoid CWE-243 and other weaknesses related to jails.
- Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright. To reduce the likelihood of code injection, use stringent allowlists that limit which constructs are allowed. If you are dynamically constructing code that invokes a function, then verifying that the input is alphanumeric might be insufficient. An attacker might still be able to reference a dangerous function that you did not intend to allow, such as system(), exec(), or exit().
- Testing: Use dynamic tools and techniques that interact with the product using large test suites with many diverse inputs, such as fuzz testing (fuzzing), robustness testing, and fault injection. The product's operation may slow down, but it should not become unstable, crash, or generate incorrect results.
- Operation: Run the code in an environment that performs automatic taint propagation and prevents any command execution that uses tainted variables, such as Perl's "-T" switch. This will force the program to perform validation steps that remove the taint, although you must be careful to correctly validate your inputs so that you do not accidentally mark dangerous inputs as untainted (see CWE-183 and CWE-184).
- Operation: Run the code in an environment that performs automatic taint propagation and prevents any command execution that uses tainted variables, such as Perl's "-T" switch. This will force the program to perform validation steps that remove the taint, although you must be careful to correctly validate your inputs so that you do not accidentally mark dangerous inputs as untainted (see CWE-183 and CWE-184).
- Implementation: For Python programs, it is frequently encouraged to use the ast.literal_eval() function instead of eval, since it is intentionally designed to avoid executing code. However, an adversary could still cause excessive memory or stack consumption via deeply nested structures [REF-1372], so the python documentation discourages use of ast.literal_eval() on untrusted data [REF-1373].
References
Advisories
Other links
Timeline
- Published
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| GHSA-xjw5-q542-3vmr Grav: config_denied_paths default list omits `system`, exposing real secrets (e.g. system.cache.redis.password) via the Twig sandbox when config_access is enabled | High | Also affects getgrav/grav |
| GHSA-3jhr-mxmx-38cx Grav: UserInterface offsetget/offsetexists allow-listed in Twig sandbox let editor-authored content leak hashed_password and 2FA secrets via offsetGet() | Medium | Also affects getgrav/grav |
| GHSA-9ccq-2jfg-qw33 Grav: Origin validation bypass in Uri::referrer() and Pages::referrerRoute() via unanchored prefix match | Critical | Also affects getgrav/grav |
| GHSA-38p6-h87p-r4cg Grav: Non constant time nonce comparison in Utils::verifyNonce() used for CSRF protection | Medium | Also affects getgrav/grav |
| GHSA-p597-crqc-m349 Grav: The system, site, and theme Twig variables bypass the content sandbox entirely and are never covered by config_denied_paths | Medium | Also affects getgrav/grav |
| GHSA-47ch-6w46-6xm7 Grav: media_directory() Twig function allows filesystem path traversal and file content disclosure from sandboxed page content | Medium | Also affects getgrav/grav |
| GHSA-jq29-c7v8-rg55 Grav: Path Traversal in MediaUploadTrait::deleteFile() Allows Arbitrary File Deletion | High | Also affects getgrav/grav |
| GHSA-8hgv-xc77-jmcr Grav: Page editors can inject arbitrary script into rendered pages via the Twig sandbox's assets.addJs/addCss allowlist, escalating to super-admin | Medium | Also affects getgrav/grav |
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