CVE-2026-82455 in RubyGems
Summary
by MITRE • 08/29/2026
RubyGems fails to re-validate path containment after filesystem symlink resolution during gem extraction. When a pre-existing symlink inside the destination directory points outside the extraction root, extracted files that appear to be written under the destination directory can instead be written outside of it, breaking the extraction safety boundary. The fix resolves the real path of the parent directory before writing and raises Gem::Package::PathError if it escapes the destination directory.
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Analysis
by VulDB Data Team • 08/29/2026
The vulnerability described involves a critical flaw in RubyGems related to how gem package files are extracted onto the local filesystem, specifically concerning the handling of symbolic links during the extraction process. This issue represents a classic path traversal or arbitrary file write scenario where the security boundary defined by the installation directory is bypassed due to insufficient validation of resolved paths after symlink resolution. In standard software distribution mechanisms like RubyGems, packages are extracted into a designated destination directory to isolate them from other system files and user data. The integrity of this isolation relies on ensuring that every file created during extraction remains strictly within the bounds of that target directory. However, in this specific implementation flaw, the validation logic fails to account for filesystem-level symbolic links that may already exist within the destination tree or are introduced via the package contents themselves.
When a gem archive contains entries that reference symlinks pointing outside the intended installation root, and these symlinks pre-exist on the target system or are created during extraction before final path validation occurs, the file writing operation can succeed in locations beyond the expected scope. The core technical failure lies in the timing of the security check; if the software validates the logical path string without resolving it to its actual physical location via realpath operations prior to writing data, an attacker can exploit symlinks to redirect writes to arbitrary directories on the filesystem. This breaks the extraction safety boundary, allowing for potential overwriting of critical system files, configuration files, or other sensitive user data located outside the gem installation directory. Such behavior is particularly dangerous because it allows a maliciously crafted gem package to perform actions that appear legitimate based on superficial path inspection but result in significant lateral movement and privilege escalation capabilities within the host environment.
From an industry standard perspective, this vulnerability aligns closely with CWE-22: Improper Limitation of a Pathname to a Restricted Directory, commonly known as directory traversal. The failure to resolve real paths before validation is also indicative of CWE-59: Improper Link Resolution Before File Access, where the system relies on symbolic links that can be manipulated by an attacker or are inherently ambiguous due to race conditions or pre-existing state. In terms of attack vectors and tactics, this flaw facilitates techniques associated with MITRE ATT&CK T1036: Masquerading, as malicious code is disguised within a legitimate-looking gem package, and potentially T1547: Boot or Log Autostart Execution if the overwritten files include startup scripts or configuration files that are executed by the operating system. The ability to write outside the sandboxed directory effectively neutralizes the containment strategy employed by the package manager, turning a routine software update mechanism into an arbitrary file write vulnerability.
The operational impact of this flaw is severe for any environment relying on RubyGems for dependency management, particularly in shared hosting environments or CI/CD pipelines where automated gem installation occurs without rigorous manual oversight. An attacker who can influence the contents of a gem package uploaded to a public registry could craft a payload that exploits this symlink resolution gap. Upon execution by an unsuspecting user or system process with sufficient permissions, the malicious gem would write files to arbitrary locations on the disk. This could lead to remote code execution if writable directories containing executable scripts are targeted, such as /usr/bin or user home directory startup configurations. It also poses a risk of data integrity compromise and potential denial of service by overwriting essential system libraries or configuration files required for normal operation.
To mitigate this vulnerability, the primary remediation involves ensuring that all file paths are resolved to their real physical locations before any write operations are performed. As noted in the fix description, resolving the real path of the parent directory prior to writing ensures that symbolic links are dereferenced and validated against the intended destination root. If the resolved real path falls outside the allowed extraction boundary, the operation should be aborted with a clear error, such as Gem::Package::PathError. Beyond this specific code fix, organizations should enforce strict input validation on all file paths derived from untrusted sources like package archives. Implementing allow-lists for permitted directories and utilizing sandboxed environments or containers during gem installation can further reduce the blast radius of any potential exploitation attempts. Regular auditing of installed gems and keeping RubyGems updated to include patches that address these path resolution flaws are essential steps in maintaining a secure software supply chain posture.