high CVSS 7.5

CVE-2026-59884·Python vulnerability

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.

Severity
high
Software
Python
Fixed in
0.6.4
Published
2026-07-14

Affected versions

Until: 0.6.4

Fixed in: 0.6.4

How to fix this CVE

Update pyasn1 to version 0.6.4 or later to fix a critical flaw in BER/CER/DER decoding that allows denial-of-service attacks through malformed ASN.1 tags. Applications that process untrusted encoded data must prioritize this update to prevent resource exhaustion and crashes. Verify the upgrade is complete by confirming the installed version matches or exceeds 0.6.4.

sudo dnf update python3-pyasn1

Defensia detects this vulnerability

How to check if you are affected

  1. Check installed pyasn1 version with: python3 -c "import pyasn1; print(pyasn1.__version__)"
  2. Verify if your application imports pyasn1 by searching code: grep -r 'from pyasn1\|import pyasn1' /path/to/app
  3. Monitor application logs for ValueError exceptions with stack traces containing 'pyasn1' or 'BER decoder'
  4. Confirm the patch by running the version check command again and verifying the output shows 0.6.4 or higher

FAQ

What is CVE-2026-59884?

This vulnerability affects pyasn1's BER/CER/DER decoder, which fails to limit the size of ASN.1 tag identifiers, allowing attackers to craft malicious input that forces expensive integer construction operations and triggers unhandled exceptions, leading to denial of service.

Is CVE-2026-59884 being actively exploited?

No, this vulnerability is not currently listed on the CISA Known Exploited Vulnerabilities (KEV) catalog and no public exploits are available, though the attack is straightforward to execute against unpatched systems.

What versions of Python are affected by CVE-2026-59884?

All versions of pyasn1 prior to 0.6.4 are affected; the vulnerability impacts Python 3.11+ more severely due to error formatting code paths, but earlier Python 3 versions are still vulnerable to resource exhaustion.

How do I check if my server is vulnerable to CVE-2026-59884?

Run: python3 -c "import pyasn1; version = pyasn1.__version__; print('Vulnerable' if tuple(map(int, version.split('.')[:3])) < (0, 6, 4) else 'Patched')"

Does Defensia detect CVE-2026-59884?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If pyasn1 is installed on a monitored server, CVE-2026-59884 will appear in your dashboard with remediation steps.

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References

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