CVE-2026-89551 SUNRPC: xdr_buf_trim: clamp buf->len to avoid underflow
An integer underflow in the Linux kernel SUNRPC module can cause buffer length corruption, potentially leading to denial of service or memory safety issues.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-89551?
CVE-2026-89551 is a critical vulnerability in Linux, published on 11 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
SUNRPC: xdr_buf_trim: clamp buf->len to avoid underflow
xdr_buf_trim() trims `len` bytes from the tail of an xdr_buf by walking the tail, pages, and head iovecs. Each per-section step uses min_t() so it never removes more bytes than that section holds, but the final accounting at the fix_len label subtracts the total bytes actually consumed from buf->len without any clamp:
fix_len: buf->len -= (len - trim);
When the caller has set buf->len to a value smaller than the sum of the iov_lens, (len - trim) can exceed buf->len and the unsigned subtraction wraps to near UINT_MAX. gss_krb5_unwrap_v2() reaches xdr_buf_trim() in exactly that state:
buf->head[0].iov_len -= GSS_KRB5_TOK_HDR_LEN + headskip; buf->len = len - (GSS_KRB5_TOK_HDR_LEN + headskip); xdr_buf_trim(buf, ec + GSS_KRB5_TOK_HDR_LEN + tailskip);
buf->len is a small wire-derived value while the iov_lens are at page scale, so the per-section loops legitimately consume far more bytes than buf->len records. The wrapped buf->len then propagates as the authoritative stream bound into every downstream XDR decoder.
Fix by clamping the decrement so buf->len bottoms out at zero:
buf->len -= min_t(unsigned int, buf->len, len - trim);
On the normal path where the iov_lens sum to buf->len, (len - trim) is always <= buf->len and the result is identical to before. No callers change behavior outside the underflow case.
Which versions are affected by CVE-2026-89551?
How do I fix CVE-2026-89551?
Apply the vendor's patch.
Is CVE-2026-89551 being exploited?
There is no known exploitation of CVE-2026-89551 in the wild and no public exploit code.
Sources
Also known as AZL-100338, BELL-CVE-2026-89551, DEBIAN-CVE-2026-89551, GHSA-7gqj-379q-cvhq, UBUNTU-CVE-2026-89551.