CVE-2026-89538

Summary

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 Software

Weaknesses

References

• https://git.kernel.org/stable/c/dfcd81ab45613d45996fbf483d4ef54b0ec90ab9
• https://git.kernel.org/stable/c/1f9856af065b6158271687e9788407b9573fd16f
• https://git.kernel.org/stable/c/880effc943ed82d6811f272ed3307c222b40f4d6
• https://git.kernel.org/stable/c/ad484748eec0a66eac0f13ab53b3fbedb7333c91

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