0010328: A logic bug in algorithm enum parsing found in `auth_digest.c` - MantisBT

ID	Project	Category	View Status	Date Submitted	Last Update

0010328	libmicrohttpd2	General	public	2025-09-02 14:23	2025-09-07 01:01

Reporter	arthurscchan	Assigned To	Karlson2k
Priority	normal	Severity	minor	Reproducibility	always
Status	resolved	Resolution	fixed
Fixed in Version	Git master

Summary	0010328: A logic bug in algorithm enum parsing found in `auth_digest.c`
Description	The issue was discovered in the function [`get_rq_dauth_algo(...)`](https://git.gnunet.org/libmicrohttpd2.git/tree/src/mhd2/auth_digest.c#n468) within `src/mhd2/auth_digest.c`. When the client’s `algorithm` parameter is quoted, several mappings return the wrong enum, diverging from the behaviour of the unquoted branch. Specifically: * `"MD5-sess"` is mapped to `MHD_DIGEST_AUTH_ALGO_SHA512_256` (should be `MHD_DIGEST_AUTH_ALGO_MD5_SESSION`). * `"SHA-512-256-sess"` is mapped to `MHD_DIGEST_AUTH_ALGO_MD5_SESSION` (should be `MHD_DIGEST_AUTH_ALGO_SHA512_256_SESSION`). * `"SHA-512-256"` (non-session) is mapped to `MHD_DIGEST_AUTH_ALGO_SHA512_256_SESSION` (should be `MHD_DIGEST_AUTH_ALGO_SHA512_256`). Other mappings are correct and consistent with the unquoted branch. ```c if (algo_param->quoted) { ... if (mhd_str_equal_caseless_quoted_s_bin_n(..., mhd_MD5_TOKEN mhd_SESS_TOKEN)) return MHD_DIGEST_AUTH_ALGO_SHA512_256; if (mhd_str_equal_caseless_quoted_s_bin_n(..., mhd_SHA512_256_TOKEN mhd_SESS_TOKEN)) return MHD_DIGEST_AUTH_ALGO_MD5_SESSION; if (mhd_str_equal_caseless_quoted_s_bin_n(..., mhd_SHA512_256_TOKEN)) return MHD_DIGEST_AUTH_ALGO_SHA512_256_SESSION; } ``` The enum `MHD_DigestAuthAlgo` is defined in [`microhttpd2.h`](https://git.gnunet.org/libmicrohttpd2.git/tree/src/include/microhttpd2.h#n7289), which confirms the correct enum values expected here: ```c enum MHD_FIXED_ENUM_MHD_APP_SET_ MHD_DigestAuthAlgo { MHD_DIGEST_AUTH_ALGO_INVALID = 0, MHD_DIGEST_AUTH_ALGO_MD5 = ..., MHD_DIGEST_AUTH_ALGO_MD5_SESSION = ..., MHD_DIGEST_AUTH_ALGO_SHA256 = ..., MHD_DIGEST_AUTH_ALGO_SHA256_SESSION = ..., MHD_DIGEST_AUTH_ALGO_SHA512_256 = ..., MHD_DIGEST_AUTH_ALGO_SHA512_256_SESSION = ..., }; ``` Each enum encodes both the base algorithm (MD5, SHA-256, SHA-512/256) and whether it is a session variant (`-sess`). The buggy code erroneously maps quoted values to different enums, breaking the intended logic of algorithm parsing. ## Consequence With quoted tokens (e.g., `algorithm="MD5-sess"`), the server misinterprets the algorithm and computing with the wrong digest, or rejecting an otherwise valid response as unsupported. This can cause interoperability problems in real world Digest authentication clients, making compliant clients appear broken.
Additional Information	Found during the ongoing security audit carried out by Ada Logics and facilitated by OSTIF in the libmicrohttpd2 project.
Tags	No tags attached.

Karlson2k 2025-09-06 16:51 manager ~0025842	The code processes unescaped token as "unquoted", so any strings like algorithm="MD5-sess" are recognised correctly. The problem could be triggered only by something exotic like algorithm="M\D5-sess". With such case the authentication still fail as the length of MD5 nonce is different from SHA256 nonce. As "-sess" algorithms are not supported, the only practical difference is another error code reported to application ("wrong nonce" instead of "unsupported algo").

Karlson2k 2025-09-06 16:52 manager ~0025843	Fixed by 10574ede9dc46eac3ff97fe514c1bb1832537fd7

Date Modified	Username	Field	Change
2025-09-02 14:23	arthurscchan	New Issue
2025-09-06 16:51	Karlson2k	Assigned To	=> Karlson2k
2025-09-06 16:51	Karlson2k	Status	new => confirmed
2025-09-06 16:51	Karlson2k	Note Added: 0025842
2025-09-06 16:52	Karlson2k	Status	confirmed => resolved
2025-09-06 16:52	Karlson2k	Resolution	open => fixed
2025-09-06 16:52	Karlson2k	Fixed in Version	=> Git master
2025-09-06 16:52	Karlson2k	Note Added: 0025843
2025-09-07 01:01	root	Project	libmicrohttpd => libmicrohttpd2
2025-09-07 01:01	root	Category	digest authentication (HTTP) => General