Summary
A path traversal vulnerability in Decap CMS's local proxy containment guard allows paths outside the intended repository root to bypass boundary checks.
An attacker who can interact with the affected local proxy may read, modify, or delete files in sibling directories whose names share the repository directory's string prefix.
A path traversal vulnerability in Decap CMS's local proxy containment guard allows paths outside the intended repository root to bypass boundary checks.
An attacker who can interact with the affected local proxy may read, modify, or delete files in sibling directories whose names share the repository directory's string prefix.
Affected context
This issue occurs in Decap CMS deployments using the local proxy before version 3.11.0, where repository paths are checked using plain string-prefix comparisons. The risk is present when a sibling directory begins with the same characters as the intended repository directory and the proxy permits requests that resolve outside the repository boundary.
This issue occurs in Decap CMS deployments using the local proxy before version 3.11.0, where repository paths are checked using plain string-prefix comparisons. The risk is present when a sibling directory begins with the same characters as the intended repository directory and the proxy permits requests that resolve outside the repository boundary.
Exploit preconditions
- Attack surface: Decap CMS local proxy file-handling functionality
- Required access: Ability to send requests to or otherwise interact with the affected proxy
- Trigger point: A file operation involving a path whose textual prefix resembles the permitted repository path
- Why exploitation is possible: The containment guard compares strings without requiring a valid path-separator boundary, allowing sibling paths to be treated as descendants of the repository root
- Attack surface: Decap CMS local proxy file-handling functionality
- Required access: Ability to send requests to or otherwise interact with the affected proxy
- Trigger point: A file operation involving a path whose textual prefix resembles the permitted repository path
- Why exploitation is possible: The containment guard compares strings without requiring a valid path-separator boundary, allowing sibling paths to be treated as descendants of the repository root
Why this happens
The underlying bug is an improper path-boundary check. A plain string-prefix comparison can consider a sibling path such as a similarly named directory to be inside the approved repository, even though its normalized filesystem location is outside that root.
This behavior maps to CWE-22: Improper Limitation of a Pathname to a Restricted Directory, because the application fails to reliably constrain filesystem operations to the intended directory.
The underlying bug is an improper path-boundary check. A plain string-prefix comparison can consider a sibling path such as a similarly named directory to be inside the approved repository, even though its normalized filesystem location is outside that root.
This behavior maps to CWE-22: Improper Limitation of a Pathname to a Restricted Directory, because the application fails to reliably constrain filesystem operations to the intended directory.
Potential impact
- An attacker may read files outside the intended repository
- An attacker could modify or delete files in sibling directories
- Repository contents or local configuration data may be exposed
- File integrity and local service availability may be affected
- In some environments, modified files could contribute to broader compromise
- An attacker may read files outside the intended repository
- An attacker could modify or delete files in sibling directories
- Repository contents or local configuration data may be exposed
- File integrity and local service availability may be affected
- In some environments, modified files could contribute to broader compromise
General mitigation guidance
Mitigation generally focuses on enforcing filesystem-aware containment rather than relying on textual comparisons.
Common defensive patterns include:
- Resolve and normalize both the requested path and repository root before comparison
- Require a path-separator-aware relationship or use trusted filesystem containment APIs
- Reject traversal segments, symlink escapes, and paths that resolve outside the approved root
- Reduce trust in client-supplied path components and construct paths from controlled identifiers
- Run the proxy with least-privilege filesystem permissions and limit access to only required directories
- Prefer secure-by-default configurations that disable local file operations when they are unnecessary
Mitigation generally focuses on enforcing filesystem-aware containment rather than relying on textual comparisons.
Common defensive patterns include:
- Resolve and normalize both the requested path and repository root before comparison
- Require a path-separator-aware relationship or use trusted filesystem containment APIs
- Reject traversal segments, symlink escapes, and paths that resolve outside the approved root
- Reduce trust in client-supplied path components and construct paths from controlled identifiers
- Run the proxy with least-privilege filesystem permissions and limit access to only required directories
- Prefer secure-by-default configurations that disable local file operations when they are unnecessary
How teams detect issues like this
Organizations can look for:
- Static analysis findings involving raw string-prefix checks for filesystem containment
- Code review signals such as concatenating user-controlled paths before normalization
- Missing canonicalization, separator-boundary checks, symlink handling, or root containment tests
- Tests covering similarly prefixed sibling directories, nested paths, traversal segments, and symbolic links
- Policy rules requiring approved path-resolution and filesystem-access abstractions
- Automated security review tools configured to identify CWE-22 path-handling patterns
Organizations can look for:
- Static analysis findings involving raw string-prefix checks for filesystem containment
- Code review signals such as concatenating user-controlled paths before normalization
- Missing canonicalization, separator-boundary checks, symlink handling, or root containment tests
- Tests covering similarly prefixed sibling directories, nested paths, traversal segments, and symbolic links
- Policy rules requiring approved path-resolution and filesystem-access abstractions
- Automated security review tools configured to identify CWE-22 path-handling patterns
