CVE-2026-87733: Mirage-crypto-ec ECDSA signature forgery via point at infinity

CVE-2026-87733 · Severity: medium · CVSS 6.2 · Published 2026-09-09

Mirage-crypto-ec is an OCaml library providing elliptic curve cryptography functions, including ECDSA digital signature verification. A flaw allows attackers to forge ECDSA signatures without possessing the private key by exploiting acceptance of the point at infinity (encoded as 0x00) as a valid public key. This undermines the authenticity guarantees of any system relying on this library for signature verification, potentially allowing unauthorized transactions, data tampering, or identity spoofing.

Technologies: OCaml Mirage-Crypto-Ec. Vendors: OCaml.

Quick answers

Is CVE-2026-87733 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-87733 being exploited in the wild.
How severe is CVE-2026-87733?
Medium severity, with a CVSS score of 6.2, published 9 September 2026.
Which products and versions are affected?
OCaml Mirage-Crypto-Ec before 2.2.0.
Is there a fix for CVE-2026-87733?
A patch was published on 2026-07-27: mirage-crypto 2.2.0 released with fix Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-87733 attacked?
It is attacked with local access, in a software library.
Am I affected by CVE-2026-87733?
Check whether you run OCaml Mirage-Crypto-Ec at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

The vulnerability is a missing cryptographic validation in the ECDSA path of mirage-crypto-ec's pub_of_octets function for P256, P384, and P521 curves. The function fails to reject 0x00 (the SEC1 encoding of the elliptic curve point at infinity), accepting it as a legitimate public key. An attacker can then forge arbitrary ECDSA signatures for any message under this crafted public key with no knowledge of a private key; the forged signature verification succeeds due to the mathematical properties of operations involving the point at infinity. The Diffie-Hellman code path correctly rejects this point via point_of_octets. Fix: Version 2.2.0 and later add a check to reject the point at infinity in pub_of_octets.

Affected products

  • OCaml mirage-crypto-ec before 2.2.0

Timeline

  • 2026-06-25: disclosed: Reported to ocaml/security-advisories
  • 2026-06-29: other: Issue acknowledged with clarifying questions
  • 2026-07-06: other: Patch provided by reporter
  • 2026-07-27: patched: mirage-crypto 2.2.0 released with fix
  • 2026-09-09: advisory: Public advisory published

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