# CVE-2026-65632: ECDSA signature length is not validated in jose, allowing signature malleability

Status: published. Published: 2026-10-26. Updated: 2026-11-02.

The jose library accepts ECDSA (ES256, ES256K, ES384, and ES512) JWS signatures of a non-canonical length. When verifying an ECDSA signature, jose_jwk_kty_ec:verify/4 in src/jwk/jose_jwk_kty_ec.erl splits the raw signature in half by its actual byte length (split_binary(Signature, byte_size(Signature) div 2)) instead of enforcing the fixed length required by RFC 7518 §3.4, and then decodes each half with crypto:bytes_to_integer/1, which ignores leading zero bytes.

As a result, an attacker who holds one valid token can prepend leading zero bytes to each of the R and S halves to produce longer signatures that still split evenly and decode to the same (r, s) pair, so many distinct signature byte strings all verify against the same key (signature malleability). This does not allow forging new payloads or impersonating a signer, because the underlying (r, s) values are unchanged. It does defeat any security control that assumes a valid token or its signature is unique, such as token-revocation-by-hash, replay or idempotency caches, and deduplication logic.

This issue affects jose: from 1.0.0 before 1.11.13.

## Remediation

- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: EEF) — https://github.com/potatosalad/erlang-jose/commit/72ce1866622b9329347dc52a94173ac0317bc9cf

## Affected products and versions

- potatosalad / jose (jose): ≥ 1.0.0; < 1.11.13 (affected, semver). Source: CNA: EEF
- potatosalad / jose (potatosalad/erlang-jose): ≥ a7746ecd4950f85a393136e449a91dac62b5c429; < 72ce1866622b9329347dc52a94173ac0317bc9cf (affected, git). Source: CNA: EEF

## Severity and prioritization

- CVSS 4.0: 5.1 (medium); CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N; source: CNA: EEF

- CVSS 4.0: 5.1 (medium); CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X; source: NVD: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db

- CISA KEV: No listing in collected sources

- EPSS: 0.09% as of 2026-11-02; percentile 0.50%

- SSVC: exploitation poc; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-347

## Exploit evidence

- exploit: https://github.com/potatosalad/erlang-jose/security/advisories/GHSA-44hw-rpw9-mwg4 (ADP: CISA-ADP)

## References

- https://github.com/potatosalad/erlang-jose/security/advisories/GHSA-44hw-rpw9-mwg4 (CNA: EEF; vendor-advisory, related)

- https://cna.erlef.org/cves/CVE-2026-65632.html (CNA: EEF; related)

- https://osv.dev/vulnerability/EEF-CVE-2026-65632 (CNA: EEF; related)

- https://github.com/potatosalad/erlang-jose/commit/a7746ecd4950f85a393136e449a91dac62b5c429 (CNA: EEF; related)

- https://github.com/potatosalad/erlang-jose/commit/72ce1866622b9329347dc52a94173ac0317bc9cf (CNA: EEF; patch)

- https://github.com/potatosalad/erlang-jose/security/advisories/GHSA-44hw-rpw9-mwg4 (ADP: CISA-ADP; exploit)

- https://cna.erlef.org/cves/CVE-2026-65632.html (6b3ad84c-e1a6-4bf7-a703-f496b71e49db)

- https://github.com/potatosalad/erlang-jose/commit/72ce1866622b9329347dc52a94173ac0317bc9cf (6b3ad84c-e1a6-4bf7-a703-f496b71e49db)

- https://github.com/potatosalad/erlang-jose/commit/a7746ecd4950f85a393136e449a91dac62b5c429 (6b3ad84c-e1a6-4bf7-a703-f496b71e49db)

- https://github.com/potatosalad/erlang-jose/security/advisories/GHSA-44hw-rpw9-mwg4 (6b3ad84c-e1a6-4bf7-a703-f496b71e49db)

- https://osv.dev/vulnerability/EEF-CVE-2026-65632 (6b3ad84c-e1a6-4bf7-a703-f496b71e49db)

- https://github.com/potatosalad/erlang-jose/security/advisories/GHSA-44hw-rpw9-mwg4 (134c704f-9b21-4f2e-91b3-4a467353bcc0)

## Provenance

- CNA: EEF: https://www.cve.org/CVERecord?id=CVE-2026-65632; retrieved 2026-11-02; source updated 2026-11-02

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-65632; retrieved 2026-11-03; source updated 2026-11-02

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-65632; retrieved 2026-11-02; source updated Not reported

Revision: cd0348b0b6b8d28ec49a83e87202b75db2a2e8169a1c69872db768354aeab1af

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