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CVE-2026-59884 pyasn1 CVE debrief

CVE-2026-59884 is a vulnerability in pyasn1, a generic ASN.1 library for Python. The BER decoder shared by the CER and DER codecs has a flaw that allows a crafted input to force construction of an arbitrarily large integer, leading to unhandled ValueError exceptions in Python 3.11+ error formatting paths. This issue is fixed in version 0.6.4. Affected applications include those that decode untrusted BER, CER, or DER input using pyasn1 versions prior to 0.6.4.

Vendor
pyasn1
Product
Unknown
CVSS
HIGH 7.5
CISA KEV
Not listed in stored evidence
Original CVE published
2026-07-14
Original CVE updated
2026-07-21
Advisory published
2026-07-14
Advisory updated
2026-07-21

Who should care

Any application decoding untrusted BER, CER, or DER input using pyasn1 versions prior to 0.6.4 is affected. This includes applications that use pyasn1 for ASN.1 decoding, especially those handling untrusted input from external sources.

Technical summary

The pyasn1 library for Python has a vulnerability in its BER decoder, which is shared by the CER and DER codecs. The decoder accumulates continuation octets without an upper bound on the tag ID size, allowing a crafted input to construct an arbitrarily large integer. This leads to CPU cost growing quadratically and triggers unhandled ValueError exceptions in Python 3.11+ error formatting paths. The issue is fixed in version 0.6.4. Developers should ensure that their applications are using the updated version of pyasn1.

Defensive priority

High priority due to the high CVSS score of 7.5 and the potential for quadratic CPU cost growth.

Recommended defensive actions

  • Inventory and check pyasn1 versions in use
  • Upgrade pyasn1 to version 0.6.4 or later
  • Implement compensating controls to monitor and limit input to pyasn1
  • Review and update application code to handle untrusted input securely
  • Monitor for suspicious activity related to BER, CER, or DER input
  • Perform regular security audits to identify potential vulnerabilities
  • Review and update incident response plans to address potential exploitation

Evidence notes

The CVE record was published on 2026-07-14T17:17:14.750Z and was last modified on 2026-07-21T14:37:52.160Z. The NVD entry is currently Analyzed. The vulnerability affects applications decoding untrusted BER, CER, or DER input using pyasn1 versions prior to 0.6.4. Evidence is limited to public sources and may not reflect the full scope or impact of the vulnerability.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-59884 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-59884

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-59884 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-59884

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.