From a5389cb63c50de603e3bbf64384971ff7f982244 Mon Sep 17 00:00:00 2001 From: Willy Tarreau Date: Mon, 29 Sep 2025 18:34:11 +0200 Subject: [PATCH] BUG/CRITICAL: mjson: fix possible DoS when parsing numbers Mjson comes with its own strtod() implementation for portability reasons and probably also because many generic strtod() versions as provided by operating systems do not focus on resource preservation and may call malloc(), which is not welcome in a parser. The strtod() implementation used here apparently originally comes from https://gist.github.com/mattn/1890186 and seems to have purposely omitted a few parts that were considered as not needed in this context (e.g. skipping white spaces, or setting errno). But when subject to the relevant test cases of the designated file above, the current function provides the same results. The aforementioned implementation uses pow() to calculate exponents, but mjson authors visibly preferred not to introduce a libm dependency and replaced it with an iterative loop in O(exp) time. The problem is that the exponent is not bounded and that this loop can take a huge amount of time. There's even an issue already opened on mjson about this: https://github.com/cesanta/mjson/issues/59. In the case of haproxy, fortunately, the watchdog will quickly stop a runaway process but this remains a possible denial of service. A first approach would consist in reintroducing pow() like in the original implementation, but if haproxy is built without Lua nor 51Degrees, -lm is not used so this will not work everywhere. Anyway here we're dealing with integer exponents, so an easy alternate approach consists in simply using shifts and squares, to compute the exponent in O(log(exp)) time. Not only it doesn't introduce any new dependency, but it turns out to be even faster than the generic pow() (85k req/s per core vs 83.5k on the same machine). This must be backported as far as 2.4, where mjson was introduced. Many thanks to Oula Kivalo for reporting this issue. CVE-2025-11230 was assigned to this issue. (cherry picked from commit 06675db4bf234ed17e14305f1d59259d2fe78b06) Signed-off-by: Christopher Faulet --- src/mjson.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/src/mjson.c b/src/mjson.c index 73b7a57..2a4106b 100644 --- a/src/mjson.c +++ b/src/mjson.c @@ -767,11 +767,13 @@ static double mystrtod(const char *str, char **end) { /* exponential part */ if ((*p == 'E') || (*p == 'e')) { + double exp, f; int i, e = 0, neg = 0; p++; if (*p == '-') p++, neg++; if (*p == '+') p++; while (is_digit(*p)) e = e * 10 + *p++ - '0'; + i = e; if (neg) e = -e; #if 0 if (d == 2.2250738585072011 && e == -308) { @@ -785,8 +787,16 @@ static double mystrtod(const char *str, char **end) { goto done; } #endif - for (i = 0; i < e; i++) d *= 10; - for (i = 0; i < -e; i++) d /= 10; + /* calculate f = 10^i */ + exp = 10; + f = 1; + while (i > 0) { + if (i & 1) f *= exp; + exp *= exp; + i >>= 1; + } + if (e > 0) d *= f; + else if (e < 0) d /= f; a = p; } else if (p > str && !is_digit(*(p - 1))) { a = str; -- 1.7.10.4