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HighVulnerabilityCVE-2026-119197updated 30 Oct 2026, 23:07 UTC

pyasn1 slows to a crawl when decoding certain numbers, tying up the CPU

A crafted number inside data sent to the pyasn1 library can make a program freeze or run very slowly while it reads that data. Someone could use this to keep a server busy, which can make it unresponsive to other users.

Fixed in pyasn1 0.6.5 · out Official, 2 sources
SeverityHighCVSS 7.5 of 10
Being exploited?UnclearNo reports either way yet
Fix available?Yespyasn1 0.6.5 · since 30 Oct 2026
How sure is this?Official, 2 sourcesBacked by an official source

How the attack works simplified

  1. 1Crafted inputLarge-mantissa REAL value in ASN.1 data
  2. 2App decodes dataBER, CER or DER decode() is called
  3. 3CPU busyQuadratic loop runs on one core
  4. 4Service slowsWorker is held for seconds or longer
  5. ResultAvailability loss from CPU exhaustion

What happened

In pyasn1 0.6.4 and earlier, the binary REAL decoder (used by BER, CER and DER decoding) builds large mantissas one byte at a time with repeated shifts and byte-string slicing, making decode time quadratic in mantissa length.

Why it is dangerous: A comparatively small crafted REAL value can keep a decoding worker busy on one CPU core for seconds or longer, causing availability loss (denial of service) for applications that decode untrusted ASN.1 data.

Are you affected?

You are affected if you run pyasn1 0.6.4 or earlier and decode attacker-controlled ASN.1 containing REAL values with the BER, CER or DER decoders, with or without a schema.

Package / productAffected versionsFixed in
pyasn1 · pip<= 0.6.40.6.5

What to do

  1. Upgrade pyasn1 to version 0.6.5 or later (pip install --upgrade pyasn1)
  2. Check your dependencies for pyasn1 and make sure they pick up the fixed version
  3. Where possible, limit the size of input data your application accepts before decoding it

Timeline

  1. 29 Oct 2026, 20:17 UTC
    Publicly disclosed
  2. 29 Oct 2026, 20:17 UTC
    Reported by NVD · NVD
  3. 29 Oct 2026, 20:21 UTC
    Picked up by Zero Radar
  4. 30 Oct 2026, 23:07 UTC
    GitHub Advisory reported on it · GitHub Advisory
  5. 30 Oct 2026, 23:07 UTC
    Fixed version published: pyasn1 0.6.5 · GitHub Advisory

Sources 2 reports, oldest first

Official vendor, government CERT or advisory database

Technical details

ID
CVE-2026-119197 · GHSA-mm57-4v3p-rchx
CVSS
7.5 / 10
Disclosed
29 Oct 2026, 20:17 UTC
On Zero Radar
29 Oct 2026, 20:21 UTC (4 min after disclosure)
Last change
30 Oct 2026, 23:07 UTC

The plain-language parts and the attack diagram were written by an AI model (anthropic/claude-haiku-5.5) from the sources above. Facts like versions, exploitation and dates come from the sources themselves. How Zero Radar works

pyasn1 slows to a crawl when decoding certain numbers, tying up the CPU · Zero Radar