CVE-2026-89538
Published: September 15, 2026Last modified: September 28, 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.
Severity score breakdown
| Parameter | Value |
|---|---|
| Base score | 9.8 |
| Attack Vector | NETWORK |
| Attack complexity | LOW |
| Privileges required | NONE |
| User interaction | NONE |
| Scope | UNCHANGED |
| Confidentiality | HIGH |
| Integrity impact | HIGH |
| Availability impact | HIGH |
| Vector | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H |
Status
| Product | Release | Package | Status |
|---|---|---|---|
| Alpaquita Linux | 23 LTS | linux-lts | Vulnerable (6.1.182-r0) |
| 25 LTS | linux-lts | Fixed (6.12.111-r0) | |
| Stream | linux-lts | Fixed (6.18.53-r0) |
References
- https://git.kernel.org/stable/c/1f9856af065b6158271687e9788407b9573fd16f
- https://git.kernel.org/stable/c/4abb44a29bb515df6f49123848765c70fc9b5021
- https://git.kernel.org/stable/c/880effc943ed82d6811f272ed3307c222b40f4d6
- https://git.kernel.org/stable/c/9fc6d6e6db49ab55208d5dd1c424c1d0d9aa7296
- https://git.kernel.org/stable/c/ac9efc29346b6942e71f0e24a3178a9121330fab
- https://git.kernel.org/stable/c/ad484748eec0a66eac0f13ab53b3fbedb7333c91
- https://git.kernel.org/stable/c/b5746095b59f4d11f39f08751d36bbdfd1e5deac
- https://git.kernel.org/stable/c/dfcd81ab45613d45996fbf483d4ef54b0ec90ab9