CVE-2026-47271 pam_usb: OOM guards removed by -DNDEBUG cause NULL dereference and authentication process crash
An attacker can cause a local denial of service by triggering a NULL pointer dereference in the pam_usb module during authentication, locking users out of sudo and login.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-47271?
CVE-2026-47271 is a medium-severity vulnerability in mcdope pam_usb, published on 27 May 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
pam_usb provides hardware authentication for Linux using ordinary removable media. Prior to 0.9.0, src/mem.c implemented out-of-memory guards for xmalloc(), xrealloc(), and xstrdup() using assert(data != NULL). The C standard specifies that all assert() expressions are compiled out when NDEBUG is defined at build time. NDEBUG is commonly defined in release and packaging builds (Debian, Fedora, Arch package flags all define it via -DNDEBUG in CFLAGS). With the guard removed, xmalloc/xrealloc/xstrdup silently return NULL on allocation failure. Every caller in the codebase dereferences the return value without a NULL check -- this is the intended design, as the guard was supposed to abort before the dereference. With the guard gone, any allocation failure causes a NULL pointer dereference, crashing the PAM module. A crash in a PAM module loaded by sudo or login causes authentication to fail for the duration of the crash, creating a local denial-of-service condition. An attacker who can induce memory pressure at authentication time can lock all users out of sudo and login. This vulnerability is fixed in 0.9.0.
Weakness: CWE-476
Which versions are affected by CVE-2026-47271?
How do I fix CVE-2026-47271?
No fix has been published for CVE-2026-47271 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-47271 being exploited?
There is no known exploitation of CVE-2026-47271 in the wild and no public exploit code.
Sources
Also known as GHSA-7rvx-jcc6-7hqq.