CVE-2021-26091
Published: 24 March 2025
Summary
CVE-2021-26091 is a high-severity PRNG (CWE-338) vulnerability in Fortinet Fortimail. Its CVSS base score is 7.5 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked in the top 49.2% of CVEs by exploit likelihood; it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified are NIST 800-53 IA-5 (Authenticator Management) and SC-13 (Cryptographic Protection).
Deeper analysis
CVE-2021-26091 is a use of a cryptographically weak pseudo-random number generator vulnerability, classified under CWE-338, affecting the authenticator in the Identity Based Encryption service of FortiMail versions 6.4.0 through 6.4.4 and 6.2.0 through 6.2.7. Published on 2025-03-24T16:15:16.450, it carries a CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N), indicating high confidentiality impact with no integrity or availability disruption.
An unauthenticated attacker with network access can exploit this vulnerability due to its low attack complexity and lack of required privileges or user interaction. Exploitation may allow the attacker to infer portions of users' authentication tokens, enabling credential resets for affected accounts.
The FortiGuard advisory FG-IR-21-031 (https://fortiguard.com/advisory/FG-IR-21-031) details mitigations and patches for this issue.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2021-12912
Vulnerability details
A use of a cryptographically weak pseudo-random number generator vulnerability in the authenticator of the Identity Based Encryption service of FortiMail 6.4.0 through 6.4.4, and 6.2.0 through 6.2.7 may allow an unauthenticated attacker to infer parts of users authentication tokens…
more
and reset their credentials.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
The CVE describes a remotely exploitable weakness in the authentication token generation of a public-facing FortiMail service, allowing unauthenticated network attackers to infer tokens and perform credential resets, which directly aligns with exploiting public-facing applications.
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Directly requires secure management and generation of authentication tokens using cryptographically strong methods, preventing inference due to weak PRNG.
Mandates use of FIPS-validated cryptographic modules with strong random number generators for protections like those in the Identity Based Encryption authenticator.
Requires identification, reporting, and correction of flaws such as the weak PRNG in FortiMail's authenticator, enabling timely patching.