PYSEC-2026-2263

pyasn1 is a generic ASN.1 library for Python. Prior to 0.6.3, the pyasn1 library is vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding ASN.1 data with deeply nested structures. An attacker can supply a crafted payload containing thousands of nested SEQUENCE (0x30) or SET (0x31) tags with "Indefinite Length" (0x80) markers. This forces the decoder to recursively call itself until the Python interpreter crashes with a RecursionError or consumes all available memory (OOM), crashing the host application. This is a distinct vulnerability from CVE-2026-23490 (which addressed integer overflows in OID decoding). The fix for CVE-2026-23490 (MAX_OID_ARC_CONTINUATION_OCTETS) does not mitigate this recursion issue. Version 0.6.3 fixes this specific issue.