CVE-2026-89542 SUNRPC: harden gss_krb5_unwrap_v2 against short tokens
A missing length check in the Linux kernel's SUNRPC GSS-KRB5 unwrap function allows out-of-bounds reads and integer overflows, potentially leading to system crashes or code execution.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-89542?
CVE-2026-89542 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: harden gss_krb5_unwrap_v2 against short tokens
gss_krb5_unwrap_v2() reads the EC and RRC header fields at ptr+4 and ptr+6 before validating that the token is at least GSS_KRB5_TOK_HDR_LEN (16) bytes long, and its rotate_left() helper passes buf->len - base to xdr_buf_subsegment() without verifying that base <= buf->len. When a caller hands in a sub-16-byte token, or a token whose declared len leaves base past the end of the buffer, three distinct failures follow:
gss_krb5_unwrap_v2(offset, len, buf) ptr = buf->head[0].iov_base + offset ec = *(ptr + 4) /* OOB read on short head */ rrc = *(ptr + 6) /* OOB read on short head */ rotate_left(offset + 16, buf, rrc) xdr_buf_subsegment(buf, &subbuf, base, buf->len - base) /* u32 wrap when base > len */ _rotate_left(&subbuf, shift) shift %= buf->len /* divide-by-zero when base == len */
After decryption, the cleanup arithmetic has the same shape:
movelen = min_t(unsigned int, buf->head[0].iov_len, len); movelen -= offset + GSS_KRB5_TOK_HDR_LEN + headskip; BUG_ON(offset + GSS_KRB5_TOK_HDR_LEN + headskip + movelen > buf->head[0].iov_len);
The BUG_ON re-adds the value just subtracted, so it reduces to min(A, B) > A and is permanently false; it cannot catch the unsigned underflow of movelen, which then drives a ~UINT_MAX-byte memmove().
Add four defense-in-depth guards inside the unwrap core so it is safe regardless of what its callers validate:
- reject tokens with len - offset < GSS_KRB5_TOK_HDR_LEN before touching ptr+4/ptr+6; - bail from rotate_left() when buf->len <= base, covering both the underflow and zero-length cases; - return early from _rotate_left() when buf->len is zero, so the shift %= buf->len modulo cannot fault; - replace the dead BUG_ON with a live check that returns GSS_S_DEFECTIVE_TOKEN before the movelen subtraction.
Which versions are affected by CVE-2026-89542?
How do I fix CVE-2026-89542?
Apply the vendor's patch.
Is CVE-2026-89542 being exploited?
There is no known exploitation of CVE-2026-89542 in the wild and no public exploit code.
Sources
Also known as AZL-100515, BELL-CVE-2026-89542, DEBIAN-CVE-2026-89542, GHSA-h928-qw5v-x6vj, UBUNTU-CVE-2026-89542.