altsvc: continue after unknown parameters

[arhxam]

RFC 7838 requires unknown Alt-Svc parameters to be ignored. The parser instead
tried to parse every parameter value as a number and stopped at nonnumeric
extension values, which could silently hide a later ma or persist.

Parse extension names up to the existing Alt-Svc line limit, and parse token
and quoted-string values before validating the two known numeric parameters.
This lets parsing continue after long extension names and quoted values
containing escaped quotes or semicolons.

The regression test verifies the cached expiry, persistence flag, and
destination after both extension value forms. I also ran the complete local
test harness: all 1,671 executable tests passed (2,063 considered).

AI assistance was used for initial code exploration and an independent diff
review; the regression was reproduced and the focused and full curl test
suites were run locally.

[bagder]

Nice find and work! I found some minor tweaks that could be done to the code and I propose this additional fix:

1. trim blanks even when quoted
2. no need to check for blanks when the value is trimmed
3. check for keywords only once and store which one it is and use that later
4. moved two variable declarations to be more local

From bf0419cb5fe9c63147db7a3d2e6b7e30a29e00dc Mon Sep 17 00:00:00 2001
From: Daniel Stenberg <daniel@haxx.se>
Date: Sun, 23 Aug 2026 17:29:04 +0200
Subject: [PATCH] tweaks

---
 lib/altsvc.c | 27 +++++++++++++--------------
 1 file changed, 13 insertions(+), 14 deletions(-)

diff --git a/lib/altsvc.c b/lib/altsvc.c
index c5dfcd192e..c57d3f6b40 100644
--- a/lib/altsvc.c
+++ b/lib/altsvc.c
@@ -469,10 +469,12 @@ static void altsvc_flush(struct altsvcinfo *asi,
       altsvc_free(as);
     }
   }
 }
 
+#define ALTSVC_MA      1
+#define ALTSVC_PERSIST 2
 static void altsvc_parse_params(const char **pp,
                                 time_t *pmaxage,
                                 bool *ppersist)
 {
   curlx_str_passblanks(pp);
@@ -480,46 +482,43 @@ static void altsvc_parse_params(const char **pp,
     return;
 
   for(;;) {
     struct Curl_str name;
     struct Curl_str val;
-    const char *vp;
-    const char *vend;
     curl_off_t num;
-    bool known;
+    int keyword = 0;
 
     /* allow some extra whitespaces around name and value */
     if(curlx_str_until(pp, &name, MAX_ALTSVC_LINE, '=') ||
        curlx_str_single(pp, '='))
       break;  /* skip further parameter parsing */
 
     curlx_str_trimblanks(&name);
-    known = curlx_str_casecompare(&name, "ma") ||
-      curlx_str_casecompare(&name, "persist");
-
     curlx_str_passblanks(pp);
     if(**pp == '\"') {
       if(curlx_str_quotedword(pp, &val, MAX_ALTSVC_LINE))
         break;
     }
     else {
       if(curlx_str_cspn(pp, &val, ",;\r\n"))
         break;
-      curlx_str_trimblanks(&val);
     }
+    curlx_str_trimblanks(&val);
+
+    if(curlx_str_casecompare(&name, "ma"))
+      keyword = ALTSVC_MA;
+    else if(curlx_str_casecompare(&name, "persist"))
+      keyword = ALTSVC_PERSIST;
 
-    if(known) {
-      vp = curlx_str(&val);
-      vend = vp + curlx_strlen(&val);
+    if(keyword) {
+      const char *vp = curlx_str(&val);
+      const char *vend = vp + curlx_strlen(&val);
       if(curlx_str_number(&vp, &num, TIME_T_MAX))
         break; /* not a number, skip further parameter parsing */
-      while((vp < vend) && ISBLANK(*vp))
-        vp++;
       if(vp != vend)
         break; /* not entirely a number, skip further parameter parsing */
-
-      if(curlx_str_casecompare(&name, "ma"))
+      if(keyword == ALTSVC_MA)
         *pmaxage = (time_t)num;
       else if(num == 1)
         *ppersist = TRUE;
     }
 
-- 
2.55.0