CVE-2026-4738: 
Linux Debian vulnerability analysis and mitigation

CVE-2026-4738 is an Improper Restriction of Operations within the Bounds of a Memory Buffer (CWE-119) vulnerability in OSGeo GDAL affecting the bundled zlib infback9 module, specifically the inftree9.c file. The vulnerability stems from a pointer offset optimization issue originally identified in CVE-2016-9840 within the upstream madler/zlib library, which was not applied to GDAL's cloned copy of the code. All GDAL versions before 3.11.0 are affected. It carries a CVSS v4.0 base score of 9.4 (Critical), assigned by GovTech CSG and published on March 24, 2026 (Feedly, EUVD).

The root cause is an unsafe pointer offset optimization in frmts/zlib/contrib/infback9/inftree9.c, a cloned function derived from the madler/zlib library. This pattern was originally flagged in CVE-2016-9840 and fixed upstream via commit 6a04314 in the zlib repository, but the fix was never propagated to GDAL's bundled copy. The vulnerability (CWE-119) allows memory operations to exceed buffer boundaries during decompression processing in the infback9 module. Exploitation requires passive user interaction (e.g., opening or processing a maliciously crafted geospatial file), with no privileges required and low attack complexity over a network vector. The fix was merged into GDAL's master branch on May 1, 2025, via pull request #12244, removing the unsafe offset pointer optimization (GitHub PR #12244).

Successful exploitation can result in high confidentiality, integrity, and availability impacts on both the vulnerable component and any dependent systems. An attacker could potentially execute arbitrary code in the context of the application processing the malicious file, cause a denial of service through application crash, or read sensitive data from memory. Given GDAL's widespread use in geospatial data processing pipelines, GIS applications, and server-side map rendering services, exploitation could affect a broad range of downstream systems (Feedly, EUVD).

The CVSS v4.0 exploit maturity is rated as 'ATTACKED,' indicating active in-the-wild exploitation has been reported, though no public proof-of-concept exploit code has been confirmed. The vulnerability is marked as automatable (AU:Y), meaning attacks can be scripted at scale. The EPSS score is approximately 0.042%, reflecting a relatively low but non-negligible probability of exploitation in the near term. No specific threat actor attribution or CISA KEV catalog listing has been identified at this time (Feedly, EUVD).

1. Reconnaissance: Identify targets using GDAL versions before 3.11.0 in geospatial data processing pipelines, web map services, or GIS applications that accept user-supplied geospatial files.
2. Craft malicious file: Create a specially crafted geospatial file (e.g., a compressed raster format processed via GDAL's zlib infback9 decompression path) designed to trigger the out-of-bounds memory operation in inftree9.c.
3. Deliver payload: Submit the malicious file to the target application via a file upload interface, API endpoint, or any mechanism that causes GDAL to process the file — requiring only passive user interaction (e.g., a server-side process opening the file).
4. Trigger vulnerability: GDAL processes the file through the infback9 decompression module, triggering the unsafe pointer offset operation in inftree9.c, resulting in a memory corruption condition.
5. Achieve objective: Depending on memory layout and exploitation precision, the attacker may achieve arbitrary code execution, cause a denial of service (crash), or read out-of-bounds memory contents (GitHub PR #12244, Feedly).

• Process: Unexpected crashes or segmentation faults in GDAL-based processes (e.g., gdalinfo, gdal_translate, or application servers using GDAL libraries) when processing external geospatial files.
• Logs: Application error logs showing memory access violations, heap corruption errors, or abnormal termination signals (SIGSEGV, SIGABRT) originating from zlib/infback9 decompression routines.
• File System: Presence of unusual or malformed compressed geospatial files (e.g., GeoTIFF, HDF5, or other compressed raster formats) submitted by external users that trigger GDAL processing errors.
• Network: Repeated submission of the same or similar malformed files from a single source IP to a GDAL-backed file processing endpoint, potentially indicating automated exploitation attempts.

The primary remediation is to upgrade OSGeo GDAL to version 3.11.0 or later, which includes the fix merged via pull request #12244 on May 1, 2025. For organizations unable to patch immediately, restrict network access to systems running affected GDAL installations, particularly those that accept externally supplied geospatial files for processing. Additionally, monitor application logs for crashes or errors originating from the zlib/infback9 decompression module. Note that the upstream madler/zlib repository also has an open pull request (#1235) to address the same issue in the original library (GitHub PR #12244, Feedly).

The vulnerability was noted on Bluesky via the CVE tracking account shortly after disclosure. INCIBE-CERT (Spain's national cybersecurity incident response team) published an early warning advisory. The fix was quietly merged into GDAL's master branch in May 2025, nearly a year before the CVE was formally assigned, with the reporter (npt-1707) notifying the GDAL maintainer of their intent to file a CVE in November 2025. Community discussion on GitHub noted that the same issue should also be addressed in the upstream madler/zlib repository, leading to an open pull request there as well (GitHub PR #12244, INCIBE).