imagecodecs 2026.3.6 through 2026.10.10 Infinite Loop via ccittfax4_decode()
Affects imagecodecs 2026.3.6 through 2026.10.10, tested on CPython 3.14 with x86-64 Linux, and only when the decoder is given a crafted 8-byte CCITT G4 input. No fix is listed in the checked facts.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Last modified
- Where the record stands: published, rejected by its publisher, or superseded by another id.What do these mean? →
- Published
- Whether CISA lists it in its Known Exploited Vulnerabilities catalog: proof that attackers are using it.What do these mean? →
- Not listed in CISA KEV
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 17/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Specific setups only
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
Summary
The imagecodecs Python library (a package for encoding and decoding image formats) and the numcodecs Ccittfax4 codec are affected. A specially crafted small CCITT Group 4 image stream can cause the decoding function to loop endlessly, pinning the CPU and hanging any program that processes untrusted input until it is forcibly terminated.
Read the original description
imagecodecs 2026.3.6 through 2026.10.10 contains an infinite loop vulnerability in ccitt_fax4_decode_size() that allows attackers to hang the calling process by supplying a crafted CCITT Group 4 stream. Attackers can provide an 8-byte G4 stream lacking EOFB to ccittfax4_decode() or the numcodecs Ccittfax4 codec, consuming full CPU until the process is killed.
What to do
Check whether your setup matches what the bug needs.
Affects imagecodecs 2026.10.10 (CPython 3.14, x86-64 Linux were the tested setup) The decoder must be given a crafted 8-byte CCITT G4 inputWorkaround: The checked facts give no workaround. The only condition they state is that the hang needs a crafted 8-byte CCITT G4 input, and no other mitigation is listed.
Timeline
- CVE published
- First seen by cve.now
Affected versions
- cgohlke imagecodecs
From 2026.3.6 · through 2026.10.10
Evidence 4
- Report
- Repository
- Repository
- Link
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-120251
What does this mean?
Passing a crafted 8-byte CCITT G4 input to ccittfax4_decode() makes the call loop forever instead of returning. The calling process then uses a full CPU until it is killed, which is a denial of service.Source: github.com
Am I affected, and how do I check?
The affected range is imagecodecs 2026.3.6 through 2026.10.10. The tested setup was CPython 3.14 on x86-64 Linux. To check a installation, look up its imagecodecs version, for example with pip show imagecodecs.Source: github.com
What should I do about it?
The checked facts do not name a fixed version, so no upgrade target is given. Every version from 2026.3.6 through 2026.10.10 is in the affected range.Source: github.com
Is it being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data
What if there's no patch?
The checked facts give no workaround. The only condition they state is that the hang needs a crafted 8-byte CCITT G4 input, and no other mitigation is listed.Source: github.com