Executive brief
GnuPG is a widely used suite of tools for encrypting and signing data. A vulnerability in its S/MIME component, gpgsm, allows an attacker to bypass security checks by providing a shorter-than-expected authentication tag. While difficult to exploit, this could theoretically allow an attacker to tamper with encrypted messages without being detected.
Technical details
A vulnerability exists in the CMS parsing logic of gpgsm within GnuPG through version 2.5.20. The component improperly validates the 'aes-ICVlen' parameter in AES-GCM AuthEnvelopedData, trusting the length of the MAC field provided in the ciphertext. While the specification recommends a 12-byte tag, the underlying libgcrypt library permits tags as short as 4 bytes. An attacker can exploit this by providing a truncated tag, reducing the integrity protection to a 32-bit value, which can be brute-forced to forge or modify encrypted messages. This is a local attack with high complexity as it requires the ability to submit numerous forgery attempts to the victim's gpgsm process.
Affected products
- GnuPG GnuPG through 2.5.20
Timeline
- 2026-06-23: disclosed: CVE published to NVD