Description
The "%xx" URL decoding function in Squid 2.5STABLE4 and earlier allows remote attackers to bypass url_regex ACLs via a URL with a NULL ("%00") character, which causes Squid to use only a portion of the requested URL when comparing it against the access control lists.
Fixed in Linux distributions
Distributions backport fixes, so the version that fixes this vulnerability differs per release — upgrade to at least the version shown. From the distributions' security trackers and advisories.
Debian
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian 14 | squid | 2.5.5-1 | — |
| Debian 13 | squid | 2.5.5-1 | — |
| Debian 12 | squid | 2.5.5-1 | — |
| Debian 3.0 | squid | 2.4.6-2woody2 | DSA-474 |
Red Hat
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| RHEL 3 | squid | 7:2.5.STABLE3-5.3E | RHSA-2004:133 |
Public exploits
Exploit or proof-of-concept code for this vulnerability is publicly available, which lowers the bar for attackers. Treat it as a reason to patch sooner.
Third-party code, not reviewed by us. Public PoCs are sometimes fake or contain malware. Read the code first and run it only in an isolated lab, against systems you are authorised to test.
Exploit-DB 1
- Squid Proxy 2.4/2.5 - NULL URL Character Unauthorized Access EDB-23777 · remote · Mar 1, 2004 · verified
Sources: PoC-in-GitHub, Metasploit Framework, Exploit-DB — updated daily. Only links are listed; no exploit code is hosted here.
Advisories 2
- RHSA-2004:133 Red Hat Security Advisory: squid security update Red Hat Security Advisories (RHSA) · Jun 26, 2026
- DSA-474 squid - ACL bypass Debian Security (DSA / DLA) · Mar 9, 2026
References
- http://www.squid-cache.org/Advisories/SQUID-2004_1.txt
- squid-urlregex-acl-bypass(15366) vdb-entry
- DSA-474 vendor-advisory
- oval:org.mitre.oval:def:877 vdb-entry signature
- 9778 vdb-entry
- MDKSA-2004:025 vendor-advisory
- oval:org.mitre.oval:def:941 vdb-entry signature
- RHSA-2004:133 vendor-advisory
- RHSA-2004:134 vendor-advisory
- 20040401 [OpenPKG-SA-2004.008] OpenPKG Security Advisory (squid) mailing-list
- GLSA-200403-11 vendor-advisory
- CLA-2004:838 vendor-advisory
- 5916 vdb-entry