EUVD-2026-34287Severity by source
AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:L
Primary rating from NVD · only source for this CVE.
CVSS VectorNVD
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:L
Lifecycle Timeline
5Description PRE-NVD
AnalysisAI
Chunk-smuggling in libpng-apng's push-mode APNG parser allows a remote unauthenticated attacker to cause integrity violations and availability disruption when a victim processes a specially crafted APNG file. The flaw - classified under CWE-436 (Interpretation Conflict) - stems from incorrect sequencing of CRC finalization and frame sequence number validation for fcTL and fdAT chunks in pngpread.c, enabling maliciously ordered chunk boundaries to bypass expected parser state transitions. No public exploit code exists at time of analysis and this vulnerability is not listed in the CISA KEV catalog; however, it represents a meaningful risk for any image-processing pipeline or application accepting user-supplied APNG data from untrusted sources.
Technical ContextAI
libpng-apng is the APNG (Animated PNG) patched variant of the canonical libpng library, which adds animation chunk handling (acTL, fcTL, fdAT) not present in stock libpng. The affected code resides in pngpread.c, which implements the push-mode (incremental/streaming) parsing API - used when image data arrives in chunks rather than all at once. CWE-436 (Interpretation Conflict) captures the root cause: prior to fix commit faf06924688b62d7c1654b5ceddedbde66ffadb4, calls to png_ensure_sequence_number (APNG frame sequence validation) and png_crc_finish (CRC checksum finalization) appeared in incorrect relative order and position within the chunk dispatch logic. This mismatch allowed a crafted file to present APNG chunks whose sequence numbers and CRC regions were validated against wrong boundaries, enabling a chunk-smuggling condition where the parser interprets chunk type or content inconsistently with what a strict reading of the PNG/APNG specification would mandate. The fix repositions these validation calls and relocates buffer-size guards inside the correct conditional branches, ensuring sequence numbers are checked and CRCs are consumed before chunk content is acted upon.
RemediationAI
Apply the upstream fix by updating to a build of libpng-apng that includes commit faf06924688b62d7c1654b5ceddedbde66ffadb4 (https://github.com/pnggroup/libpng/commit/faf06924688b62d7c1654b5ceddedbde66ffadb4); consult the GitHub security advisory GHSA-c4v6-gxrq-6g2x for the officially patched release version once confirmed. If a packaged release is not yet available for your distribution, apply the patch to pngpread.c directly, as the diff is self-contained. As a compensating control, applications that do not require animation support can switch from the push-mode API to the pull-mode API, which is not affected by this vulnerability, though this may require code changes if the streaming API is architecturally required. For internet-facing upload services, rejecting files detected as APNG (by checking for the acTL chunk) at the ingestion boundary until patched is an effective but operationally disruptive workaround, as it blocks all animated PNG processing. Sandboxing the image decoding process (e.g., running it in a seccomp-filtered subprocess) limits blast radius even if exploitation occurs.
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External POC / Exploit Code
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