High severity · published Jul 14, 2026
CVE-2026-59884
pyasn1 BER/CER/DER decoder denial of service via unbounded long-form tag IDs
7.5
CVSS
0.6%
EPSS
No
On KEV
Description
pyasn1 is a generic ASN.1 library for Python. Prior to 0.6.4, the BER decoder shared by the CER and DER codecs parses long-form tags by accumulating continuation octets without an upper bound on the tag ID size, allowing a crafted input to force construction of an arbitrarily large integer with CPU cost growing quadratically and to trigger unhandled ValueError exceptions in Python 3.11+ error formatting paths. Any application decoding untrusted BER, CER, or DER input is affected. This issue is fixed in version 0.6.4.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
None
Integrity
None
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.6% · higher than 49% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
PY
pyasn1 · pyasn1
< 0.6.4 · source: vendor, NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-59884
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-59884
Patch
https://github.com/pyasn1/pyasn1/commit/628e36ecbb5277a3f01572ce418ef54271b165a5
Release Notes
https://github.com/pyasn1/pyasn1/releases/tag/v0.6.4
Third Party Advisory
https://github.com/pyasn1/pyasn1/security/advisories/GHSA-m4p7-r5rc-7g4j
More from pyasn1
7.5
CVE-2026-59885
pyasn1: Quadratic complexity in OBJECT IDENTIFIER and RELATIVE-OID processing allows denial of service
7.5
CVE-2026-59886
pyasn1: Uncontrolled resource consumption when converting decoded REAL values
7.5
CVE-2026-30922
pyasn1 Vulnerable to Denial of Service via Unbounded Recursion
7.5
CVE-2026-23490
pyasn1 has a DoS vulnerability in decoder