From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Date: Tue, 7 Apr 2026 07:48:12 +0000 (+0200)
Subject: BUG/MEDIUM: jwt: fix heap overflow in ECDSA signature DER conversion
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=648b0e7beac4746f1f4184a2f09eedad5246fff3;p=thirdparty%2Fhaproxy.git

BUG/MEDIUM: jwt: fix heap overflow in ECDSA signature DER conversion

convert_ecdsa_sig() calls i2d_ECDSA_SIG(ecdsa_sig, &p) where p
points into signature->area, a trash chunk of tune.bufsize bytes
(default 16384). i2d writes with no output bound.

The raw R||S input can be up to bufsize bytes (filled by
base64urldec at jwt.c:520-527), giving bignum_len up to 8192. The
DER encoding adds a SEQUENCE header (2-4 bytes), two INTEGER headers
(2-4 bytes each), and up to two leading-zero sign-padding bytes when
the bignum high bit is set. With two 8192-byte bignums having the
high bit set, the encoding is ~16398 bytes, overflowing the 16384-
byte buffer by ~14 bytes.

Triggered by any JWT with alg=ES256/384/512 and a ~21830-character
base64url signature. The signature does not need to verify
successfully; the overflow happens before verification. Reachable
from any config using jwt_verify with an EC algorithm.

Also fixes the existing wrong check: i2d returns -1 on error which
became SIZE_MAX in the size_t signature->data, defeating the
"== 0" test.

This must be backported as far as JWT support exists.
---

diff --git a/src/jwt.c b/src/jwt.c
index 3dc04b1fb..0ab557afe 100644
--- a/src/jwt.c
+++ b/src/jwt.c
@@ -331,14 +331,21 @@ static int convert_ecdsa_sig(const struct jwt_ctx *ctx, struct buffer *signature
 	/* Build ecdsa out of R and S values. */
 	ECDSA_SIG_set0(ecdsa_sig, ec_R, ec_S);
 
-	p = (unsigned char*)signature->area;
-
-	signature->data = i2d_ECDSA_SIG(ecdsa_sig, &p);
-	if (signature->data == 0) {
+	/* i2d_ECDSA_SIG writes with no output bound. The DER encoding adds
+	 * a SEQUENCE header (~4 bytes), two INTEGER headers (~4 bytes each),
+	 * and up to two sign-padding bytes on top of the raw R||S bytes.
+	 * Compute the length first to avoid overflowing signature->area.
+	 */
+	retval = i2d_ECDSA_SIG(ecdsa_sig, NULL);
+	if (retval <= 0 || (size_t)retval > signature->size) {
 		retval = JWT_VRFY_INVALID_TOKEN;
 		goto end;
 	}
 
+	p = (unsigned char*)signature->area;
+	signature->data = i2d_ECDSA_SIG(ecdsa_sig, &p);
+	retval = 0;
+
 end:
 	ECDSA_SIG_free(ecdsa_sig);
 	return retval;