CVE-2026-87735: 
Linux Debian vulnerability analysis and mitigation

CVE-2026-87735 is a denial-of-service vulnerability in the mirage-crypto-pk package for OCaml, caused by an undocumented exception triggered when processing small messages during RSA decryption or encryption operations. It affects all versions of mirage-crypto-pk before 2.3.0. The vulnerability was published on September 9, 2026, and carries a CVSS v3.1 base score of 4.3 (Medium) (GitHub Advisory).

The root cause is classified as CWE-1284 (Improper Validation of Specified Quantity in Input): the library fails to validate or properly handle message size inputs before passing them to RSA cryptographic operations, resulting in an unhandled, undocumented exception when a message is unexpectedly small. An authenticated, network-based attacker with low privileges can trigger this exception by submitting a crafted small message to any service invoking RSA encryption or decryption via the affected library. No public proof-of-concept code is known to exist at this time (GitHub Advisory).

Successful exploitation causes the RSA decryption or encryption operation to raise an unhandled exception, which can crash the affected service or cause the cryptographic operation to fail entirely. The impact is limited to availability — there is no confidentiality or integrity loss — but services relying on mirage-crypto-pk for TLS or other cryptographic operations could be rendered temporarily unavailable. Lateral movement or data exfiltration are not directly facilitated by this vulnerability (GitHub Advisory).

There is no evidence of active in-the-wild exploitation, no public proof-of-concept, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. The EPSS score is approximately 0.141% (4th percentile), indicating a low probability of exploitation in the near term. NVD SSVC assessment classifies exploitation as "none" and the attack as non-automatable (GitHub Advisory).

Upgrade mirage-crypto-pk to version 2.3.0 or later, which resolves the undocumented exception. As interim mitigations, implement input validation to reject or sanitize abnormally small messages before they reach RSA operations, and add exception handling around RSA encryption/decryption calls to prevent unhandled crashes from propagating to the service layer (GitHub Advisory).