CVE-2020-37239: libbabl Use After Free Vulnerability

CVE-2020-37239 Overview

CVE-2020-37239 affects libbabl 0.1.62, a pixel encoding and color space conversion library used by GEGL and GIMP. The library implements a custom double-free detection mechanism using a signature field on allocated chunks. Attackers can bypass this safeguard because libc's malloc metadata overwrites the babl signature when memory is freed. As a result, calling babl_free() twice on the same pointer evades detection and can corrupt heap state. The flaw is classified as [CWE-415] Double Free and can lead to memory corruption and arbitrary code execution in processes that handle attacker-controlled image data.

Critical Impact
Successful exploitation enables heap memory corruption and potential remote code execution in applications consuming untrusted image data through libbabl.

Affected Products

libbabl version 0.1.62
GEGL image processing library (consumer of libbabl)
Applications linking libbabl 0.1.62 for color space conversion

Discovery Timeline

2026-05-16 - CVE-2020-37239 published to NVD
2026-05-18 - Last updated in NVD database

Technical Details for CVE-2020-37239

Vulnerability Analysis

The vulnerability resides in libbabl's custom heap protection logic. The library prepends a signature field to each allocated chunk and inspects this signature inside babl_free() to detect double-free conditions. The check fails because libc's allocator reuses the same memory region for its own free-chunk metadata once the buffer is released. When babl_free() runs a second time on the same pointer, the signature it expects has already been overwritten by glibc internal bookkeeping, and the routine cannot distinguish a freed chunk from a live one.

This design flaw breaks the central safety assumption of libbabl's allocator wrapper. Code paths that double-free an allocation now proceed without aborting, returning the same chunk to the application twice or corrupting the heap free list. The condition maps to [CWE-415] and produces classic double-free exploitation primitives.

Root Cause

The root cause is overlapping use of the same memory region by two allocators. libbabl stores its integrity signature inline with user data, while glibc places its free-chunk metadata in the same bytes after free() returns. libbabl's detection logic does not account for this overlap and trusts a value that the underlying allocator legitimately overwrites.

Attack Vector

An attacker supplies crafted input to an application that processes images through libbabl. The attacker triggers a code path that calls babl_free() twice on the same pointer. The second free corrupts the glibc heap, producing primitives that can be shaped into arbitrary write and ultimately code execution. Network reachability depends on the host application — image converters, thumbnailers, and rendering services exposed to untrusted files are the primary targets.

No verified public exploit code is reproduced here. See the VulnCheck Advisory for Libbabl and Exploit-DB #49259 for technical analysis.

Detection Methods for CVE-2020-37239

Indicators of Compromise

Crashes or SIGABRT aborts in processes linking libbabl, particularly with glibc messages such as double free or corruption and free(): invalid pointer.
Unexpected termination of GEGL, GIMP, or image conversion services when processing externally supplied files.
Heap corruption signatures in core dumps referencing babl_free or babl_calloc frames.

Detection Strategies

Run libbabl-dependent workloads under AddressSanitizer or Valgrind in test environments to surface double-free conditions before production exposure.
Inventory all installed copies of libbabl across endpoints and build pipelines, flagging any instance at version 0.1.62.
Monitor application crash telemetry for repeated faults in image-processing services that consume user-supplied content.

Monitoring Recommendations

Enable core dump collection on hosts running GEGL or GIMP and forward stack traces to a central log store for review.
Alert on process crash rates above baseline for daemons that ingest untrusted images.
Track package versions of babl and gegl through software composition analysis to detect drift back to vulnerable releases.

How to Mitigate CVE-2020-37239

Immediate Actions Required

Upgrade libbabl to a release later than 0.1.62 that removes the broken detection logic or aligns it with allocator behavior.
Rebuild and redistribute any downstream packages that statically link or vendor libbabl 0.1.62.
Restrict the file types and sources accepted by services that pass content through libbabl until patched binaries are deployed.

Patch Information

No vendor patch URL is listed in the CVE record. Refer to the GEGL Babl Resource for current releases and the VulnCheck Advisory for Libbabl for remediation guidance. Verify the installed version with the package manager and confirm replacement of all copies on the system.

Workarounds

Sandbox image-processing workloads using seccomp, AppArmor, or SELinux profiles to contain memory corruption side effects.
Disable or remove libbabl-dependent features from internet-facing services that cannot be upgraded immediately.
Enable glibc hardening options such as MALLOC_CHECK_=3 for non-performance-critical processes to abort on heap corruption.

bash

# Verify installed libbabl version and locate binaries
dpkg -l | grep -i babl
ldconfig -p | grep babl

# Enable glibc allocator checks for a wrapped process
MALLOC_CHECK_=3 /usr/bin/gegl input.png -o output.png