CVE-2026-59884

pyasn1 BER/CER/DER decoder denial of service via unbounded long-form tag IDs

Affected components

• pyasn1.codec.ber.decoder, decode() and StreamingDecoder
• pyasn1.codec.cer.decoder and pyasn1.codec.der.decoder, which inherit
  the same tag parsing
• pyasn1.type.tag, Tag/TagSet reprs could raise ValueError when
  rendering oversized tag IDs (reachable through decoder error paths)

The encoders and the pyasn1.codec.native codec are not affected.

Workarounds

Bound the size of untrusted input passed to decode() before calling it.

The BER decoder (shared by the CER and DER codecs) parses long-form tags by accumulating continuation octets in a loop with no upper bound on the size of the tag ID. A crafted input can force the decoder to build an arbitrarily large integer, with CPU cost growing quadratically in input size, a ~1 MB input consumes over a minute of CPU. On Python 3.11+, the oversized tag ID can also trigger an unhandled ValueError (integer string conversion limit) while the decoder formats error messages, violating the documented PyAsn1Error contract and potentially bypassing caller error handling.

Any application decoding untrusted BER/CER/DER input is affected.

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2026-59884 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.6.4); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

Fixed in 0.6.4. Long-form tag IDs are now limited to 20 octets (140-bit tag IDs, matching the existing OID arc limit); oversized tags are rejected with PyAsn1Error. Tag ID rendering in reprs and error messages was additionally hardened against the interpreter's integer-to-string conversion limit.

1. What is CVE-2026-59884? CVE-2026-59884 is a high-severity uncontrolled resource consumption vulnerability in pyssn1 (pip), affecting versions <= 0.6.3. It is fixed in 0.6.4. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2026-59884? CVE-2026-59884 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of pyssn1 are affected by CVE-2026-59884? pyssn1 (pip) versions <= 0.6.3 is affected.
4. Is there a fix for CVE-2026-59884? Yes. CVE-2026-59884 is fixed in 0.6.4. Upgrade to this version or later.
5. Is CVE-2026-59884 exploitable, and should I be worried? Whether CVE-2026-59884 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-59884 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-59884? Upgrade pyssn1 to 0.6.4 or later.