joserfc's PBES2 p2c Unbounded Iteration Count enables Denial of Service (DoS)

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2026-27932: 0.46% estimated exploitation activity in the next 30 days; percentile 40.06 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary A resource exhaustion vulnerability in joserfc allows an unauthenticated attacker to cause a Denial of Service (DoS) via CPU exhaustion. When the library decrypts a JSON Web Encryption (JWE) token using Password-Based Encryption (PBES2) algorithms, it reads the p2c (PBES2 Count) parameter directly from the token's protected header. This parameter defines the number of iterations for the PBKDF2 key derivation function. Because joserfc does not validate or bound this value, an attacker can specify an extremely large iteration count (e.g., 2^31 - 1), forcing the server to expend massive CPU resources processing a single token. This vulnerability exists at the JWA layer and impacts all high-level JWE and JWT decryption interfaces if PBES2 algorithms are allowed by the application's policy. Details Vulnerable file: src/joserfc/rfc7518/jwealgs.py Vulnerable function: PBES2HSAlgKeyEncryption.decryptcek() Lines: 283 The p2c value is then passed to computederivedkey : Impact on JWT…

Published by the source

2 Mar 2026, 18:47 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:36 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-w5r5-m38g-f9f9

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-w5r5-m38g-f9f9

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# Patch Next advisory brief: CVE-2026-27932

Advisory: joserfc's PBES2 p2c Unbounded Iteration Count enables Denial of Service (DoS)
IDs: CVE-2026-27932, GHSA-w5r5-m38g-f9f9
Record: https://patchnext.com/advisories/cve-2026-27932 (revision 2, source checked 26 Oct 2026, 13:36 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: joserfc 1.6.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI joserfc: affected < 1.6.3; fixed in 1.6.3

## Suggested inspection (read-only)
- `python -m pip show joserfc`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on joserfc.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-w5r5-m38g-f9f9 (OSV record GHSA-w5r5-m38g-f9f9 lists affected ranges: PyPI joserfc >=0 <1.6.3 (fixed))
- Verified: https://github.com/advisories/GHSA-w5r5-m38g-f9f9 (GitHub-reviewed advisory GHSA-w5r5-m38g-f9f9 lists affected ranges: PyPI joserfc >=0 <1.6.3)

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