dbcve.org
Ubuntu LinuxOperating system · Canonical

CVE-2020-15810

MEDIUM · 6.5 CVSS v3.1 Published 2020-09-02
Fix available
A fix is available. Upgrade to 4.13 / 5.0.4 or later.
See remediation →
71/100
Remediation priority · Elevated
Remotely reachable Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
An issue was discovered in Squid before 4.13 and 5.x before 5.0.4. Due to incorrect data validation, HTTP Request Smuggling attacks may succeed against HTTP and HTTPS traffic. This leads to cache poisoning. This allows any client, including browser scripts, to bypass local security and poison the proxy cache and any downstream caches with content from an arbitrary source. When configured for relaxed header parsing (the default), Squid relays headers containing whitespace characters to upstream servers. When this occurs as a prefix to a Content-Length header, the frame length specified will be ignored by Squid (allowing for a conflicting length to be used from another Content-Length header) but relayed upstream.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

HTTP Request Smuggling vulnerability in Squid proxy servers before 4.13 and 5.0.4 allows attackers to poison caches by exploiting incorrect validation of whitespace-prefixed Content-Length headers in relaxed header parsing mode. Attackers can send requests with conflicting Content-Length values that Squid ignores but forwards to upstream servers, enabling injection of arbitrary content into downstream caches.

MitigationUpgrade to Squid 4.13/5.0.4 or later, or disable relaxed header parsing (set 'relaxed_header_parser off') in Squid configuration if immediate upgrade is not feasible.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Ubuntu LinuxOperating system
Affected:= 16.04= 18.04= 20.04
Debian LinuxOperating system
Affected:= 9.0= 10.0
FedoraOperating system
Affected:= 31= 32= 33
SquidApplication
Affected:< 4.13>= 5.0, < 5.0.4
LeapOperating system
Affected:= 15.1= 15.2

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify Squid version
    Run 'squid -v' or 'squid -V' to display version information. On some systems, 'dpkg -l squid' or 'rpm -q squid' can also show the installed version.
    Affected if Version is less than 4.13, or version 5.0.x is less than 5.0.4 (for example, 5.0.1, 5.0.2, 5.0.3)
  2. Locate Squid configuration file
    Common paths are /etc/squid/squid.conf, /etc/squid3/squid.conf, or /etc/squid/squid.conf.default. Use 'squid -k parse 2>&1 | head -20' to see config file location in any error output.
    Affected if Configuration file exists (Squid is actively configured)
  3. Check relaxed_header_parser setting
    Search the Squid config file for 'relaxed_header_parser' using 'grep -i relaxed_header_parser /etc/squid/squid.conf' or equivalent. If nothing is returned, check default behavior in Squid documentation for your version.
    Affected if The directive is set to 'on' OR is not explicitly set to 'off' in the configuration (the vulnerability only applies when relaxed header parsing is enabled)
  4. Verify Squid service is running
    Run 'systemctl status squid' or 'service squid status' to check if Squid daemon is active. Also check for listening ports with 'ss -tlnp | grep 3128' (default Squid port).
    Affected if Squid is actively running as a proxy or cache server (the vulnerability only affects active Squid instances)
  5. Check for upstream proxy configuration
    Examine the Squid config for 'cache_peer' directives which indicate upstream proxy relationships. Run 'grep -i cache_peer /etc/squid/squid.conf'.
    Affected if Squid is configured to forward requests to upstream servers (the attack exploits the forwarding of malformed requests to upstream servers)

You are affected if Squid version is below 4.13 or between 5.0.0 and 5.0.3 inclusive, AND relaxed_header_parser is set to 'on' or not explicitly disabled, AND Squid is running as a proxy with upstream server connections.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

From vendor data
Upgrade available Upgrade to 4.13 / 5.0.4 or later
Fixed in 4.135.0.4
Interim mitigation

Upgrade to Squid 4.13/5.0.4 or later, or disable relaxed header parsing (set 'relaxed_header_parser off') in Squid configuration if immediate upgrade is not feasible.

Fix this in Ubuntu Linux Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing2.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,240
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2020-15810 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data