CVE-2026-59884 pyasn1 CVE debrief

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.

Official resources