From db87c8d9fe621539531f6f915ba9e1755a2a26cb Mon Sep 17 00:00:00 2001 From: Willy Tarreau Date: Mon, 7 Apr 2025 15:30:43 +0200 Subject: [PATCH] BUG/MEDIUM: sample: fix risk of overflow when replacing multiple regex back-refs Aleandro Prudenzano of Doyensec and Edoardo Geraci of Codean Labs reported a bug in sample_conv_regsub(), which can cause replacements of multiple back-references to overflow the temporary trash buffer. The problem happens when doing "regsub(match,replacement,g)": we're replacing every occurrence of "match" with "replacement" in the input sample, which requires a length check. For this, a max is applied, so that a replacement may not use more than the remaining length in the buffer. However, the length check is made on the replaced pattern and not on the temporary buffer used to carry the new string. This results in the remaining size to be usable for each input match, which can go beyond the temporary buffer size if more than one occurrence has to be replaced with something that's larger than the remaining room. The fix proposed by Aleandro and Edoardo is the correct one (check on "trash" not "output"), and is the one implemented in this patch. While it is very unlikely that a config will replace multiple short patterns each with a larger one in a request, this possibility cannot be entirely ruled out (e.g. mask a known, short IP address using "XXX.XXX.XXX.XXX"). However when this happens, the replacement pattern will be static, and not be user-controlled, which is why this patch is marked as medium. The bug was introduced in 2.2 with commit 07e1e3c93e ("MINOR: sample: regsub now supports backreferences"), so it must be backported to all versions. Special thanks go to Aleandro and Edoardo for reporting this bug with a simple reproducer and a fix. (cherry picked from commit 3e3b9eebf871510aee36c3a3336faac2f38c9559) Signed-off-by: Aurelien DARRAGON --- src/sample.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/src/sample.c b/src/sample.c index ab44bfe..4ea7b5f 100644 --- a/src/sample.c +++ b/src/sample.c @@ -3168,7 +3168,7 @@ static int sample_conv_regsub(const struct arg *arg_p, struct sample *smp, void output->data = exp_replace(output->area, output->size, start, arg_p[1].data.str.area, pmatch); /* replace the matching part */ - max = output->size - output->data; + max = trash->size - trash->data; if (max) { if (max > output->data) max = output->data; -- 1.7.10.4