CVE-2026-89542
CriticalCVSS 9.8Exploitation Probability (EPSS)
Low risk6th percentile - higher than 6% of all known CVEs
Summary
A vulnerability was fixed in the Linux kernel's SUNRPC gss_krb5_unwrap_v2() function, which read EC and RRC header fields out of bounds before validating the token length. Unsigned arithmetic underflow could trigger a ~UINT_MAX-byte memmove, and a zero-length buffer could cause a divide-by-zero. Four defense-in-depth guards were added to validate token and buffer lengths.
Risk Assessment
The vulnerability could be exploited by an attacker to cause a kernel crash (denial of service) or potentially achieve code execution via out-of-bounds reads and a massive memmove operation. It affects systems using Kerberos GSS over SUNRPC.
Recommendation
Update the Linux kernel to a version containing the fix for CVE-2026-89542. If updating is not possible, restrict access to SUNRPC/Kerberos services to trusted networks.
Other vulnerabilities in Linux kernel
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Original NVD description (English source)
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.