# CVE-2026-89538

## Summary

- **CVE ID:** CVE-2026-89538
- **Severity:** CRITICAL
- **CVSS Score:** 9.8 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H)
- **CWE:** N/A
- **Published:** Sep 11, 2026
- **Last Modified:** Sep 14, 2026

## Description

In the Linux kernel, the following vulnerability has been resolved:

SUNRPC: Reject krb5 v2 wrap tokens with oversized ec field

gss_krb5_unwrap_v2() sets buf->len to a logical
length, which can be much smaller than head[0].iov_len
(the allocated receive-page capacity).  It then calls
xdr_buf_trim() with a trim length derived from the 16-bit
"extra count" (ec) field in the Kerberos v2 token header.

The ec field is authenticated by the post-decrypt memcmp()
against the encrypted header copy, so a randomly-mutated
value is rejected.  However, any peer holding a valid GSS
context can legitimately encrypt a token whose ec exceeds
the plaintext length.  Per RFC 4121, such a token is
structurally malformed.

Although xdr_buf_trim() now clamps the buf->len subtraction
to avoid unsigned underflow, the buffer is still left in a
semantically invalid state (zero length, inconsistent iov
lengths) when ec is oversized.

Reject these tokens before calling xdr_buf_trim(), giving
callers a well-defined GSS_S_DEFECTIVE_TOKEN error and
keeping the xdr_buf internally consistent.  The wrapped blob
begins at a nonzero offset -- both callers pass len as
offset + opaque_len -- so buf->len still counts the offset
bytes that precede the blob.  Compare the trim length
against the remaining wrapped segment, buf->len - offset,
rather than the whole buffer; comparing against buf->len
alone leaves an offset-wide window in which an oversized ec
passes the test and xdr_buf_trim() cuts into the bytes ahead
of the blob.

## Affected Products

- Linux — Linux (cf4c024b908353fcc48309374d39e3399d67dfd1)
- Linux — Linux (3.13)
- Linux — Linux (0)
- Linux — Linux (6.12.109)
- Linux — Linux (6.18.50)
- Linux — Linux (7.2.4)
- Linux — Linux (7.3-rc1)
- Linux — Linux (5.10.270)
- Linux — Linux (5.15.221)
- Linux — Linux (6.1.188)
- Linux — Linux (6.6.157)

## References

- [CNA](https://git.kernel.org/stable/c/dfcd81ab45613d45996fbf483d4ef54b0ec90ab9)
- [CNA](https://git.kernel.org/stable/c/1f9856af065b6158271687e9788407b9573fd16f)
- [CNA](https://git.kernel.org/stable/c/880effc943ed82d6811f272ed3307c222b40f4d6)
- [CNA](https://git.kernel.org/stable/c/ad484748eec0a66eac0f13ab53b3fbedb7333c91)
- [CNA](https://git.kernel.org/stable/c/ac9efc29346b6942e71f0e24a3178a9121330fab)
- [CNA](https://git.kernel.org/stable/c/b5746095b59f4d11f39f08751d36bbdfd1e5deac)
- [CNA](https://git.kernel.org/stable/c/9fc6d6e6db49ab55208d5dd1c424c1d0d9aa7296)
- [CNA](https://git.kernel.org/stable/c/4abb44a29bb515df6f49123848765c70fc9b5021)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.70%
- **EPSS Percentile:** 51.6

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-18._