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.
---
src/jwt.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)
diff --git a/src/jwt.c b/src/jwt.c
index 3dc04b1fbb9b..0ab557afe413 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;
--
2.53.0
