CVE-2026-65632
ECDSA signature length is not validated in jose, allowing signature malleability
What happened?
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.
Affected software and versions
| Software | Version information | Fixed version |
|---|---|---|
| josepotatosalad | affected: 1.0.0 to < 1.11.13 | See vendor advisory |
| potatosalad/erlang-josepotatosalad | affected: a7746ecd4950f85a393136e449a91dac62b5c429 to < 72ce1866622b9329347dc52a94173ac0317bc9cf | See vendor advisory |
Version information comes from the source record and can include unaffected products and exceptions. Distribution packages may contain backported fixes: an older upstream version alone does not prove a server is affected.
Is it being exploited?
This CVE was not listed in the CISA catalogue at our last retrieval. This does not establish that exploitation is absent.
How to respond
- Confirm the product, installed package version, and any configuration requirements in the vendor advisory.
- Follow the vendor’s remediation or workaround guidance in the references below. Structured fixed-version data is unavailable here; the publisher’s description may include a fix or workaround.
- Restart affected services if required, then verify the running version. Known exploitation may warrant investigation as well as patching.
Sources and remediation references
- https://www.cve.org/CVERecord?id=CVE-2026-65632
- https://github.com/potatosalad/erlang-jose/security/advisories/GHSA-44hw-rpw9-mwg4
- https://cna.erlef.org/cves/CVE-2026-65632.html
- https://osv.dev/vulnerability/EEF-CVE-2026-65632
- https://github.com/potatosalad/erlang-jose/commit/a7746ecd4950f85a393136e449a91dac62b5c429
- https://github.com/potatosalad/erlang-jose/commit/72ce1866622b9329347dc52a94173ac0317bc9cf