Squid is a caching proxy for the Web. In Squid versions prior to 7.2, a failure to redact HTTP authentication credentials in error handling allows information disclosure. The vulnerability allows a script to bypass browser security protections and learn the credentials a trusted client uses to authenticate. This potentially allows a remote client to identify security tokens or credentials used internally by a web application using Squid for backend load balancing. These attacks do not require Squid to be configured with HTTP authentication. The vulnerability is fixed in version 7.2. As a workaround, disable debug information in administrator mailto links generated by Squid by configuring squid.conf with emailerrdata off.

An issue was discovered in http/ContentLengthInterpreter.cc in Squid before 4.12 and 5.x before 5.0.3. A Request Smuggling and Poisoning attack can succeed against the HTTP cache. The client sends an HTTP request with a Content-Length header containing "+\ "-" or an uncommon shell whitespace character prefix to the length field-value.

An issue was discovered in Squid through 4.7. When handling the tag esi:when when ESI is enabled, Squid calls ESIExpression::Evaluate. This function uses a fixed stack buffer to hold the expression while it's being evaluated. When processing the expression, it could either evaluate the top of the stack, or add a new member to the stack. When adding a new member, there is no check to ensure that the stack won't overflow.

An issue was discovered in Squid before 4.9. URN response handling in Squid suffers from a heap-based buffer overflow. When receiving data from a remote server in response to an URN request, Squid fails to ensure that the response can fit within the buffer. This leads to attacker controlled data overflowing in the heap.

An issue was discovered in Squid through 4.7. When handling requests from users, Squid checks its rules to see if the request should be denied. Squid by default comes with rules to block access to the Cache Manager, which serves detailed server information meant for the maintainer. This rule is implemented via urlregex. The handler for urlregex rules URL decodes an incoming request. This allows an attacker to encode their URL to bypass the urlregex check, and gain access to the blocked resource.

An issue was discovered in Squid before 5.0.2. A remote attacker can replay a sniffed Digest Authentication nonce to gain access to resources that are otherwise forbidden. This occurs because the attacker can overflow the nonce reference counter (a short integer). Remote code execution may occur if the pooled token credentials are freed (instead of replayed as valid credentials).

An issue was discovered in Squid 3.3.9 through 3.5.28 and 4.x through 4.7. When Squid is configured to use Digest authentication, it parses the header Proxy-Authorization. It searches for certain tokens such as domain, uri, and qop. Squid checks if this token's value starts with a quote and ends with one. If so, it performs a memcpy of its length minus 2. Squid never checks whether the value is just a single quote (which would satisfy its requirements), leading to a memcpy of its length minus 1.

Description: Due to chunked decoder lenience Squid is vulnerable to Request/Response smuggling attacks when parsing HTTP/1.1 and ICAP messages

Reference: https://github.com/squid-cache/squid/security/advisories/GHSA-j83v-w3p4-5cqh

Affected versions: 2.6-6.3. Patched in 6.4.

Squid is a caching proxy for the Web. Prior to version 7.5, due to heap Use-After-Free, Squid is vulnerable to Denial of Service when handling ICP traffic. This problem allows a remote attacker to perform a reliable and repeatable Denial of Service attack against the Squid service using ICP protocol. This attack is limited to Squid deployments that explicitly enable ICP support (i.e. configure non-zero icpport). This problem cannot be mitigated by denying ICP queries using icpaccess rules. Version 7.5 contains a patch.

An issue was discovered in Squid before 4.9. When handling a URN request, a corresponding HTTP request is made. This HTTP request doesn't go through the access checks that incoming HTTP requests go through. This causes all access checks to be bypassed and allows access to restricted HTTP servers, e.g., an attacker can connect to HTTP servers that only listen on localhost.

An issue was discovered in Squid 4.0.23 through 4.7. When checking Basic Authentication with HttpHeader::getAuth, Squid uses a global buffer to store the decoded data. Squid does not check that the decoded length isn't greater than the buffer, leading to a heap-based buffer overflow with user controlled data.

Reference: https://github.com/squid-cache/squid/commits/v4 http://www.squid-cache.org/Versions/v4/changesets/

Upstream Patch: http://www.squid-cache.org/Versions/v4/changesets/squid-4-7f73e9c5d17664b882ed32590e6af310c247f320.patch

Squid has Denial of Service in ICP Response handling

Last updated 25 August 2025

Description: Due to a buffer overflow bug Squid is vulnerable to a Denial of Service attack against HTTP Digest Authentication

Reference: https://github.com/squid-cache/squid/security/advisories/GHSA-phqj-m8gv-cq4g

Affected versions: 3.2.0.1-5.9, 6.0-6.3

Description: a) Due to an Incorrect Conversion between Numeric Types bug Squid is vulnerable to a Denial of Service attack against FTP Native Relay input validation.

b) Due to an Incorrect Conversion between Numeric Types bug Squid is vulnerable to a Denial of Service attack against ftp:// URL validation and access control.

Reference: https://github.com/squid-cache/squid/security/advisories/GHSA-2g3c-pg7q-g59w

Affected versions: 5.0.3-5.9, 6.0-6.3

Last updated 13 January 2025

Buffer overflow in wccp.c in Squid 2.5 before 2.5.STABLE7 allows remote attackers to cause a denial of service and possibly execute arbitrary code via a long WCCP packet, which is processed by a recvfrom function call that uses an incorrect length parameter.

Buffer overflow in the idnsALookup function in dnsinternal.cc in Squid 3.2 through 3.2.11 and 3.3 through 3.3.6 allows remote attackers to cause a denial of service (memory corruption and server termination) via a long name in a DNS lookup request.

Last updated 25 August 2025

Last updated 25 August 2025

Last updated 25 August 2025

http.cc in Squid 3.x before 3.5.15 and 4.x before 4.0.7 proceeds with the storage of certain data after a response-parsing failure, which allows remote HTTP servers to cause a denial of service (assertion failure and daemon exit) via a malformed response.

Due to incorrect pointer handling, Squid versions 3.x (prior to 3.5.27) and 4.x (prior to 4.0.23) are vulnerable to a denial of service attack when processing ESI responses. This problem allows a remote server delivering certain ESI response syntax to trigger a denial of service for all clients accessing the Squid service.

Upstream Advisory: http://www.squid-cache.org/Advisories/SQUID-20181.txt

Upstream Patches: http://www.squid-cache.org/Versions/v3/3.5/changesets/SQUID-20181.patch http://www.squid-cache.org/Versions/v4/changesets/SQUID-20181.patch

Due to incorrect pointer handling, Squid versions 3.x (prior to 3.5.27) and 4.x (prior to 4.0.23) are vulnerable to a denial of service attack when processing HTTP messages or downloading intermediate CA certificates. This problem allows a remote client delivering certain HTTP requests in conjunction with certain trusted server responses to trigger a denial of service for all clients accessing the Squid service.

Upstream Advisory: http://www.squid-cache.org/Advisories/SQUID-20182.txt

Upstream Patches: http://www.squid-cache.org/Versions/v3/3.5/changesets/SQUID-20182.patch http://www.squid-cache.org/Versions/v4/changesets/SQUID-20182.patch

An issue was discovered in Squid 3.x and 4.x through 4.8. Due to incorrect input validation, there is a heap-based buffer overflow that can result in Denial of Service to all clients using the proxy. Severity is high due to this vulnerability occurring before normal security checks; any remote client that can reach the proxy port can trivially perform the attack via a crafted URI scheme.

An issue was discovered in Squid 2.x, 3.x, and 4.x through 4.8. Due to incorrect data management, it is vulnerable to information disclosure when processing HTTP Digest Authentication. Nonce tokens contain the raw byte value of a pointer that sits within heap memory allocation. This information reduces ASLR protections and may aid attackers isolating memory areas to target for remote code execution attacks.

An issue was discovered in Squid through 4.7 and 5. When receiving a request, Squid checks its cache to see if it can serve up a response. It does this by making a MD5 hash of the absolute URL of the request. If found, it servers the request. The absolute URL can include the decoded UserInfo (username and password) for certain protocols. This decoded info is prepended to the domain. This allows an attacker to provide a username that has special characters to delimit the domain, and treat the rest of the URL as a path or query string. An attacker could first make a request to their domain using an encoded username, then when a request for the target domain comes in that decodes to the exact URL, it will serve the attacker's HTML instead of the real HTML. On Squid servers that also act as reverse proxies, this allows an attacker to gain access to features that only reverse proxies can use, such as ESI.

An issue was discovered in Squid before 4.10. It allows a crafted FTP server to trigger disclosure of sensitive information from heap memory, such as information associated with other users' sessions or non-Squid processes.

An issue was discovered in Squid before 4.10. Due to incorrect input validation, it can interpret crafted HTTP requests in unexpected ways to access server resources prohibited by earlier security filters.

Due to incorrect string termination, Squid cachemgr.cgi 4.0 through 4.7 may access unallocated memory. On systems with memory access protections, this can cause the CGI process to terminate unexpectedly, resulting in a denial of service for all clients using it.