CVE-2026-25966: ImageMagick Auth Bypass Vulnerability

CVE-2026-25966 Overview

ImageMagick, a widely-used open-source software suite for image editing and manipulation, contains a security policy bypass vulnerability that allows attackers to circumvent stdin/stdout restrictions. The vulnerability exists in how ImageMagick handles file descriptor pseudo-filenames (fd:<n> format), which are not blocked by the "secure" policy templates despite explicit rules intended to prevent reading and writing from standard streams.

Critical Impact

Attackers with local access can bypass ImageMagick's security policy protections by using fd: pseudo-filenames (e.g., fd:0, fd:1), potentially enabling unauthorized access to standard input/output streams and undermining the security goals of the "secure" policy configuration.

Affected Products

• ImageMagick versions prior to 7.1.2-15
• ImageMagick versions prior to 6.9.13-40

Discovery Timeline

• 2026-02-24 - CVE CVE-2026-25966 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-25966

Vulnerability Analysis

This vulnerability represents an Improper Access Control flaw (CWE-284) in ImageMagick's security policy implementation. The shipped "secure" security policy template includes explicit rules designed to prevent ImageMagick from reading or writing to standard input and output streams. However, the policy implementation fails to account for an alternative path specification method supported by ImageMagick.

ImageMagick supports file descriptor pseudo-filenames in the format fd:<n>, where n represents a file descriptor number. Common examples include fd:0 for standard input, fd:1 for standard output, and fd:2 for standard error. The security policy templates that block access to standard streams do not include rules to prevent access via this alternative naming convention.

This gap creates a scenario where users or processes can bypass intended security restrictions by specifying file descriptors using the fd: format rather than conventional stdin/stdout references. The impact is particularly concerning in environments where ImageMagick is used in processing pipelines with untrusted input, as the security policy is specifically designed to prevent potentially dangerous stream-based operations.

Root Cause

The root cause is an incomplete security policy implementation that blocks standard stream access through conventional path references but fails to account for the fd:<n> pseudo-filename format. This represents an allowlist/blocklist gap where an alternative syntax for accessing the same resources was not included in the security controls.

Attack Vector

This vulnerability requires local access to exploit. An attacker with the ability to invoke ImageMagick commands or influence ImageMagick processing parameters can leverage the fd: pseudo-filename format to bypass security policy restrictions. By specifying fd:0, fd:1, or other file descriptor numbers, the attacker can read from or write to streams that the security policy was designed to protect.

The attack is particularly relevant in scenarios where ImageMagick is deployed with the "secure" policy template to process untrusted images or where command-line arguments can be influenced by external input. The vulnerability effectively nullifies the stdin/stdout protection that administrators expect when deploying the secure policy configuration.

Detection Methods for CVE-2026-25966

Indicators of Compromise

• ImageMagick commands or log entries containing fd:0, fd:1, fd:2, or similar fd:<n> patterns
• Unusual stream-based operations in ImageMagick processing pipelines
• Unexpected data flow through standard input/output in ImageMagick contexts
• Process execution anomalies where ImageMagick accesses file descriptors unexpectedly

Detection Strategies

• Monitor ImageMagick command-line arguments and configuration files for fd: pseudo-filename patterns
• Implement application-level logging to capture file path arguments passed to ImageMagick functions
• Review security policy configurations to verify explicit blocking of fd: format paths
• Audit scripts and applications that invoke ImageMagick for potential injection of fd: paths

Monitoring Recommendations

• Enable verbose logging for ImageMagick operations in production environments
• Implement process monitoring to detect unexpected stdin/stdout access by ImageMagick processes
• Configure SIEM alerts for patterns matching fd:\d+ in command-line arguments or log files
• Establish baseline behavior for ImageMagick usage and alert on deviations

How to Mitigate CVE-2026-25966

Immediate Actions Required

• Upgrade ImageMagick to version 7.1.2-15 or later (for 7.x branch)
• Upgrade ImageMagick to version 6.9.13-40 or later (for 6.x branch)
• Review and update security policy configurations to explicitly block fd: pseudo-filenames
• Audit existing deployments for potential exploitation attempts

Patch Information

ImageMagick has released patched versions that include updated secure policy templates with explicit blocking of fd: pseudo-filenames. The fix is available in versions 7.1.2-15 and 6.9.13-40. For detailed patch information, refer to the GitHub Security Advisory.

Workarounds

• Manually update the ImageMagick security policy file (policy.xml) to include explicit rules blocking fd: pseudo-filename patterns
• Implement input validation in applications that pass file paths to ImageMagick to reject fd: format strings
• Use application-level sandboxing to restrict ImageMagick's access to file descriptors
• Consider using container isolation to limit the impact of potential policy bypasses

# Configuration example - Add to policy.xml to block fd: pseudo-filenames
# Location: /etc/ImageMagick-7/policy.xml or /etc/ImageMagick-6/policy.xml

# Add the following policy rule to prevent fd: pseudo-filename access:
# <policy domain="path" rights="none" pattern="fd:*" />