CVE-2026-35611

Affected Versions

This vulnerability affects Addressable >= 2.3.0 (note: 2.3.0 and 2.3.1 were yanked; the earliest installable release is 2.3.2). It was partially fixed in version 2.8.10 and fully remediated in 2.9.0.

The vulnerability is more exploitable on MRI Ruby < 3.2 and on all versions of JRuby and TruffleRuby. MRI Ruby 3.2 and later ship with Onigmo 6.9, which introduces memoization that prevents catastrophic backtracking for the first class of template. JRuby and TruffleRuby do not implement equivalent memoization and remain vulnerable to all patterns.

This has been confirmed on the following runtimes:

Workarounds

• Upgrade to MRI Ruby 3.2 or later, if your application does not use JRuby or TruffleRuby. The Onigmo memoization introduced in MRI Ruby 3.2 prevents catastrophic backtracking from nested unbounded quantifiers (pattern 1 above, templates using the * modifier). It does not reliably mitigate the O(n^k) multi-variable case (pattern 2), so upgrading Ruby alone may not be sufficient if your templates use {+v1,v2,...} or {#v1,v2,...} syntax.

• Avoid using vulnerable template patterns when matching user-supplied input on unpatched versions of the library:

  • Templates using the * (explode) modifier: {foo*}, {+var*}, {#var*}, {.var*}, {/var*}, {;var*}, {?var*}, {&var*}
  • Templates using multiple variables with the + or # operators: {+v1,v2}, {#v1,v2,v3}, etc.

• Apply a short timeout around any call to Template#match or Template#extract that processes user-supplied data.

References

• https://owasp.org/www-community/attacks/Regular_expression_Denial_of_Service_-_ReDoS
• https://cwe.mitre.org/data/definitions/1333.html
• https://www.regular-expressions.info/catastrophic.html

Credits

Discovered in collaboration with @jamfish.

For more information

If you have any questions or comments about this advisory:

• Open an issue

Within the URI template implementation in Addressable, two classes of URI template generate regular expressions vulnerable to catastrophic backtracking:

1. Templates using the * (explode) modifier with any expansion operator (e.g., {foo*}, {+var*}, {#var*}, {/var*}, {.var*}, {;var*}, {?var*}, {&var*}) generate patterns with nested unbounded quantifiers that are O(2^n) when matched against a maliciously crafted URI.
2. Templates using multiple variables with the + or # operators (e.g., {+v1,v2,v3}) generate patterns with O(n^k) complexity due to the comma separator being within the matched character class, causing ambiguous backtracking across k variables.

When matched against a maliciously crafted URI, this can result in catastrophic backtracking and uncontrolled resource consumption, leading to denial of service. The first pattern was partially addressed in 2.8.10 for certain operator combinations. Both patterns are fully remediated in 2.9.0.

Users of the URI parsing capabilities in Addressable but not the URI template matching capabilities are unaffected.

A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use. Typical impact: denial of service when input is crafted to trigger backtracking.

CVE-2026-35611 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.9.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-35611? CVE-2026-35611 is a high-severity inefficient regular expression (ReDoS) vulnerability in addressable (rubygems), affecting versions >= 2.3.0, < 2.9.0. It is fixed in 2.9.0. A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use.
2. How severe is CVE-2026-35611? CVE-2026-35611 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of addressable are affected by CVE-2026-35611? addressable (rubygems) versions >= 2.3.0, < 2.9.0 is affected.
4. Is there a fix for CVE-2026-35611? Yes. CVE-2026-35611 is fixed in 2.9.0. Upgrade to this version or later.
5. Is CVE-2026-35611 exploitable, and should I be worried? Whether CVE-2026-35611 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-35611 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-35611? Upgrade addressable to 2.9.0 or later.