Executive brief
A vulnerability in OpenSSH's client software could allow a malicious server to crash the client application. This occurs when the client is running in FIPS mode and connects to a server that provides specifically crafted security parameters. While this primarily results in a service disruption (Denial of Service) for the user or automated process initiating the connection, it does not currently appear to allow for data theft or unauthorized access.
Technical details
A double free vulnerability exists in the `input_kex_dh_gex_group()` function within `kexgexc.c` of OpenSSH. When the client is operating in FIPS mode, it performs a known-group validation on server-supplied parameters. If this validation fails, the code jumps to a cleanup routine that frees BIGNUM objects (`p` and `g`) that have already been assigned to the `kex->dh` structure. A second free occurs during the subsequent teardown of the SSH state in `kex_free()`. This is most reliably triggered in non-fatal client flows like `ssh-keyscan`. Exploitation requires the client to negotiate `diffie-hellman-group-exchange-sha256` and receive attacker-controlled DH-GEX parameters. The primary impact is a crash (SIGABRT) of the client process.
Affected products
- OpenSSH OpenSSH 9.9p1-22.el10_2 (Red Hat build)
- Red Hat Red Hat Enterprise Linux 6
- Red Hat Red Hat Enterprise Linux 7
- Red Hat Red Hat Enterprise Linux 8
- Red Hat Red Hat Enterprise Linux 9
- Red Hat Red Hat Enterprise Linux 10
- Red Hat Red Hat OpenShift Container Platform 4
Timeline
- 2026-04-26: disclosed: Initial report to Red Hat Bugzilla
- 2026-06-23: advisory: CVE published by NVD/Red Hat