GHSA-3R47-Q2HR-FJXP
Vulnerability from github – Published: 2026-09-11 21:31 – Updated: 2026-09-14 15:32In 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": [],
"aliases": [
"CVE-2026-89538"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-09-11T20:19:36Z",
"severity": "CRITICAL"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nSUNRPC: Reject krb5 v2 wrap tokens with oversized ec field\n\ngss_krb5_unwrap_v2() sets buf-\u003elen to a logical\nlength, which can be much smaller than head[0].iov_len\n(the allocated receive-page capacity). It then calls\nxdr_buf_trim() with a trim length derived from the 16-bit\n\"extra count\" (ec) field in the Kerberos v2 token header.\n\nThe ec field is authenticated by the post-decrypt memcmp()\nagainst the encrypted header copy, so a randomly-mutated\nvalue is rejected. However, any peer holding a valid GSS\ncontext can legitimately encrypt a token whose ec exceeds\nthe plaintext length. Per RFC 4121, such a token is\nstructurally malformed.\n\nAlthough xdr_buf_trim() now clamps the buf-\u003elen subtraction\nto avoid unsigned underflow, the buffer is still left in a\nsemantically invalid state (zero length, inconsistent iov\nlengths) when ec is oversized.\n\nReject these tokens before calling xdr_buf_trim(), giving\ncallers a well-defined GSS_S_DEFECTIVE_TOKEN error and\nkeeping the xdr_buf internally consistent. The wrapped blob\nbegins at a nonzero offset -- both callers pass len as\noffset + opaque_len -- so buf-\u003elen still counts the offset\nbytes that precede the blob. Compare the trim length\nagainst the remaining wrapped segment, buf-\u003elen - offset,\nrather than the whole buffer; comparing against buf-\u003elen\nalone leaves an offset-wide window in which an oversized ec\npasses the test and xdr_buf_trim() cuts into the bytes ahead\nof the blob.",
"id": "GHSA-3r47-q2hr-fjxp",
"modified": "2026-09-14T15:32:28Z",
"published": "2026-09-11T21:31:31Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-89538"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/1f9856af065b6158271687e9788407b9573fd16f"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/4abb44a29bb515df6f49123848765c70fc9b5021"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/880effc943ed82d6811f272ed3307c222b40f4d6"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/9fc6d6e6db49ab55208d5dd1c424c1d0d9aa7296"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/ac9efc29346b6942e71f0e24a3178a9121330fab"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/ad484748eec0a66eac0f13ab53b3fbedb7333c91"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/b5746095b59f4d11f39f08751d36bbdfd1e5deac"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/dfcd81ab45613d45996fbf483d4ef54b0ec90ab9"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
"type": "CVSS_V3"
}
]
}
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